We are currently solving the following issue within our repository. Here is the issue text:
--- BEGIN ISSUE ---
Add nullable attribute to the Team Model
Following changes are required in the Team model: 

- [x] Convert the URLFields to CharField with URL

- [x] Change the fields `email`, `github_url`, `linkedin_url`, `personal_website` to nullable fields

--- END ISSUE ---

Below are some code segments, each from a relevant file. One or more of these files may contain bugs.
--- BEGIN FILES ---
Path: `apps/web/models.py`
Content:
```
1 from __future__ import unicode_literals
2 
3 from django.db import models
4 
5 from base.models import (TimeStampedModel, )
6 
7 
8 class Contact(TimeStampedModel):
9     """Model representing details of User submitting queries."""
10     name = models.CharField(max_length=100,)
11     email = models.EmailField(max_length=70,)
12     message = models.CharField(max_length=500,)
13 
14     def __unicode__(self):
15         return "%s: %s: %s" % (self.name, self.email, self.message)
16 
17     class Meta:
18         app_label = 'web'
19         db_table = 'contact'
20 
21 
22 class Team(models.Model):
23     """Model representing details of Team"""
24 
25     # Team Type Options
26     CORE_TEAM = 'Core Team'
27     CONTRIBUTOR = 'Contributor'
28 
29     TEAM_TYPE_OPTIONS = (
30         (CORE_TEAM, CORE_TEAM),
31         (CONTRIBUTOR, CONTRIBUTOR),
32     )
33 
34     name = models.CharField(max_length=100)
35     email = models.EmailField(max_length=70, null=True)
36     description = models.TextField(null=True)
37     headshot = models.ImageField(upload_to="headshots", null=True, blank=True)
38     visible = models.BooleanField(default=True)
39     github_url = models.CharField(max_length=200, null=True)
40     linkedin_url = models.CharField(max_length=200, null=True)
41     personal_website = models.CharField(max_length=200, null=True)
42     background_image = models.ImageField(upload_to="bg-images", null=True, blank=True)
43     team_type = models.CharField(choices=TEAM_TYPE_OPTIONS, max_length=50)
44 
45     def __unicode__(self):
46         return self.name
47 
48     class Meta:
49         app_label = 'web'
50         db_table = 'teams'
51
```

Path: `apps/web/migrations/0004_change_team_model_field_type.py`
Content:
```
1 # -*- coding: utf-8 -*-
2 # Generated by Django 1.10.2 on 2017-03-19 21:06
3 from __future__ import unicode_literals
4 
5 from django.db import migrations, models
6 
7 
8 class Migration(migrations.Migration):
9 
10     dependencies = [
11         ('web', '0003_added_description_and_background_image_to_team_model'),
12     ]
13 
14     operations = [
15         migrations.AlterField(
16             model_name='team',
17             name='email',
18             field=models.EmailField(max_length=70, null=True),
19         ),
20         migrations.AlterField(
21             model_name='team',
22             name='github_url',
23             field=models.CharField(max_length=200, null=True),
24         ),
25         migrations.AlterField(
26             model_name='team',
27             name='linkedin_url',
28             field=models.CharField(max_length=200, null=True),
29         ),
30         migrations.AlterField(
31             model_name='team',
32             name='personal_website',
33             field=models.CharField(max_length=200, null=True),
34         ),
35     ]
36
```
--- END FILES ---

Please first localize the bug based on the issue statement, and then generate a patch according to the `git diff` format fenced by three backticks.

Here is an example:

```diff
diff --git a/examples/server_async.py b/examples/server_async.py
--- a/examples/server_async.py
+++ b/examples/server_async.py
@@ -313,4 +313,4 @@
 
 
 if __name__ == "__main__":
-    asyncio.run(run_async_server("."), debug=True)
+    asyncio.run(run_async_server(), debug=True)
diff --git a/examples/server_sync.py b/examples/server_sync.py
--- a/examples/server_sync.py
+++ b/examples/server_sync.py
@@ -313,5 +313,5 @@
 
 
 if __name__ == "__main__":
-    server = run_sync_server(".")
+    server = run_sync_server()
     server.shutdown()

```

diff --git a/apps/web/migrations/0004_change_team_model_field_type.py b/apps/web/migrations/0004_change_team_model_field_type.py
--- a/apps/web/migrations/0004_change_team_model_field_type.py
+++ b/apps/web/migrations/0004_change_team_model_field_type.py
@@ -1,5 +1,5 @@
 # -*- coding: utf-8 -*-
-# Generated by Django 1.10.2 on 2017-03-19 21:06
+# Generated by Django 1.10.2 on 2017-03-19 21:58
 from __future__ import unicode_literals
 
 from django.db import migrations, models
@@ -15,21 +15,21 @@
         migrations.AlterField(
             model_name='team',
             name='email',
-            field=models.EmailField(max_length=70, null=True),
+            field=models.EmailField(blank=True, max_length=70, null=True),
         ),
         migrations.AlterField(
             model_name='team',
             name='github_url',
-            field=models.CharField(max_length=200, null=True),
+            field=models.CharField(blank=True, max_length=200, null=True),
         ),
         migrations.AlterField(
             model_name='team',
             name='linkedin_url',
-            field=models.CharField(max_length=200, null=True),
+            field=models.CharField(blank=True, max_length=200, null=True),
         ),
         migrations.AlterField(
             model_name='team',
             name='personal_website',
-            field=models.CharField(max_length=200, null=True),
+            field=models.CharField(blank=True, max_length=200, null=True),
         ),
     ]
diff --git a/apps/web/models.py b/apps/web/models.py
--- a/apps/web/models.py
+++ b/apps/web/models.py
@@ -32,13 +32,13 @@
     )
 
     name = models.CharField(max_length=100)
-    email = models.EmailField(max_length=70, null=True)
+    email = models.EmailField(max_length=70, null=True, blank=True)
     description = models.TextField(null=True)
     headshot = models.ImageField(upload_to="headshots", null=True, blank=True)
     visible = models.BooleanField(default=True)
-    github_url = models.CharField(max_length=200, null=True)
-    linkedin_url = models.CharField(max_length=200, null=True)
-    personal_website = models.CharField(max_length=200, null=True)
+    github_url = models.CharField(max_length=200, null=True, blank=True)
+    linkedin_url = models.CharField(max_length=200, null=True, blank=True)
+    personal_website = models.CharField(max_length=200, null=True, blank=True)
     background_image = models.ImageField(upload_to="bg-images", null=True, blank=True)
     team_type = models.CharField(choices=TEAM_TYPE_OPTIONS, max_length=50)

{"golden_diff": "diff --git a/apps/web/migrations/0004_change_team_model_field_type.py b/apps/web/migrations/0004_change_team_model_field_type.py\n--- a/apps/web/migrations/0004_change_team_model_field_type.py\n+++ b/apps/web/migrations/0004_change_team_model_field_type.py\n@@ -1,5 +1,5 @@\n # -*- coding: utf-8 -*-\n-# Generated by Django 1.10.2 on 2017-03-19 21:06\n+# Generated by Django 1.10.2 on 2017-03-19 21:58\n from __future__ import unicode_literals\n \n from django.db import migrations, models\n@@ -15,21 +15,21 @@\n         migrations.AlterField(\n             model_name='team',\n             name='email',\n-            field=models.EmailField(max_length=70, null=True),\n+            field=models.EmailField(blank=True, max_length=70, null=True),\n         ),\n         migrations.AlterField(\n             model_name='team',\n             name='github_url',\n-            field=models.CharField(max_length=200, null=True),\n+            field=models.CharField(blank=True, max_length=200, null=True),\n         ),\n         migrations.AlterField(\n             model_name='team',\n             name='linkedin_url',\n-            field=models.CharField(max_length=200, null=True),\n+            field=models.CharField(blank=True, max_length=200, null=True),\n         ),\n         migrations.AlterField(\n             model_name='team',\n             name='personal_website',\n-            field=models.CharField(max_length=200, null=True),\n+            field=models.CharField(blank=True, max_length=200, null=True),\n         ),\n     ]\ndiff --git a/apps/web/models.py b/apps/web/models.py\n--- a/apps/web/models.py\n+++ b/apps/web/models.py\n@@ -32,13 +32,13 @@\n     )\n \n     name = models.CharField(max_length=100)\n-    email = models.EmailField(max_length=70, null=True)\n+    email = models.EmailField(max_length=70, null=True, blank=True)\n     description = models.TextField(null=True)\n     headshot = models.ImageField(upload_to=\"headshots\", null=True, blank=True)\n     visible = models.BooleanField(default=True)\n-    github_url = models.CharField(max_length=200, null=True)\n-    linkedin_url = models.CharField(max_length=200, null=True)\n-    personal_website = models.CharField(max_length=200, null=True)\n+    github_url = models.CharField(max_length=200, null=True, blank=True)\n+    linkedin_url = models.CharField(max_length=200, null=True, blank=True)\n+    personal_website = models.CharField(max_length=200, null=True, blank=True)\n     background_image = models.ImageField(upload_to=\"bg-images\", null=True, blank=True)\n     team_type = models.CharField(choices=TEAM_TYPE_OPTIONS, max_length=50)\n", "issue": "Add nullable attribute to the Team Model\nFollowing changes are required in the Team model: \r\n\r\n- [x] Convert the URLFields to CharField with URL\r\n\r\n- [x] Change the fields `email`, `github_url`, `linkedin_url`, `personal_website` to nullable fields\n", "before_files": [{"content": "from __future__ import unicode_literals\n\nfrom django.db import models\n\nfrom base.models import (TimeStampedModel, )\n\n\nclass Contact(TimeStampedModel):\n    \"\"\"Model representing details of User submitting queries.\"\"\"\n    name = models.CharField(max_length=100,)\n    email = models.EmailField(max_length=70,)\n    message = models.CharField(max_length=500,)\n\n    def __unicode__(self):\n        return \"%s: %s: %s\" % (self.name, self.email, self.message)\n\n    class Meta:\n        app_label = 'web'\n        db_table = 'contact'\n\n\nclass Team(models.Model):\n    \"\"\"Model representing details of Team\"\"\"\n\n    # Team Type Options\n    CORE_TEAM = 'Core Team'\n    CONTRIBUTOR = 'Contributor'\n\n    TEAM_TYPE_OPTIONS = (\n        (CORE_TEAM, CORE_TEAM),\n        (CONTRIBUTOR, CONTRIBUTOR),\n    )\n\n    name = models.CharField(max_length=100)\n    email = models.EmailField(max_length=70, null=True)\n    description = models.TextField(null=True)\n    headshot = models.ImageField(upload_to=\"headshots\", null=True, blank=True)\n    visible = models.BooleanField(default=True)\n    github_url = models.CharField(max_length=200, null=True)\n    linkedin_url = models.CharField(max_length=200, null=True)\n    personal_website = models.CharField(max_length=200, null=True)\n    background_image = models.ImageField(upload_to=\"bg-images\", null=True, blank=True)\n    team_type = models.CharField(choices=TEAM_TYPE_OPTIONS, max_length=50)\n\n    def __unicode__(self):\n        return self.name\n\n    class Meta:\n        app_label = 'web'\n        db_table = 'teams'\n", "path": "apps/web/models.py"}, {"content": "# -*- coding: utf-8 -*-\n# Generated by Django 1.10.2 on 2017-03-19 21:06\nfrom __future__ import unicode_literals\n\nfrom django.db import migrations, models\n\n\nclass Migration(migrations.Migration):\n\n    dependencies = [\n        ('web', '0003_added_description_and_background_image_to_team_model'),\n    ]\n\n    operations = [\n        migrations.AlterField(\n            model_name='team',\n            name='email',\n            field=models.EmailField(max_length=70, null=True),\n        ),\n        migrations.AlterField(\n            model_name='team',\n            name='github_url',\n            field=models.CharField(max_length=200, null=True),\n        ),\n        migrations.AlterField(\n            model_name='team',\n            name='linkedin_url',\n            field=models.CharField(max_length=200, null=True),\n        ),\n        migrations.AlterField(\n            model_name='team',\n            name='personal_website',\n            field=models.CharField(max_length=200, null=True),\n        ),\n    ]\n", "path": "apps/web/migrations/0004_change_team_model_field_type.py"}], "after_files": [{"content": "from __future__ import unicode_literals\n\nfrom django.db import models\n\nfrom base.models import (TimeStampedModel, )\n\n\nclass Contact(TimeStampedModel):\n    \"\"\"Model representing details of User submitting queries.\"\"\"\n    name = models.CharField(max_length=100,)\n    email = models.EmailField(max_length=70,)\n    message = models.CharField(max_length=500,)\n\n    def __unicode__(self):\n        return \"%s: %s: %s\" % (self.name, self.email, self.message)\n\n    class Meta:\n        app_label = 'web'\n        db_table = 'contact'\n\n\nclass Team(models.Model):\n    \"\"\"Model representing details of Team\"\"\"\n\n    # Team Type Options\n    CORE_TEAM = 'Core Team'\n    CONTRIBUTOR = 'Contributor'\n\n    TEAM_TYPE_OPTIONS = (\n        (CORE_TEAM, CORE_TEAM),\n        (CONTRIBUTOR, CONTRIBUTOR),\n    )\n\n    name = models.CharField(max_length=100)\n    email = models.EmailField(max_length=70, null=True, blank=True)\n    description = models.TextField(null=True)\n    headshot = models.ImageField(upload_to=\"headshots\", null=True, blank=True)\n    visible = models.BooleanField(default=True)\n    github_url = models.CharField(max_length=200, null=True, blank=True)\n    linkedin_url = models.CharField(max_length=200, null=True, blank=True)\n    personal_website = models.CharField(max_length=200, null=True, blank=True)\n    background_image = models.ImageField(upload_to=\"bg-images\", null=True, blank=True)\n    team_type = models.CharField(choices=TEAM_TYPE_OPTIONS, max_length=50)\n\n    def __unicode__(self):\n        return self.name\n\n    class Meta:\n        app_label = 'web'\n        db_table = 'teams'\n", "path": "apps/web/models.py"}, {"content": "# -*- coding: utf-8 -*-\n# Generated by Django 1.10.2 on 2017-03-19 21:58\nfrom __future__ import unicode_literals\n\nfrom django.db import migrations, models\n\n\nclass Migration(migrations.Migration):\n\n    dependencies = [\n        ('web', '0003_added_description_and_background_image_to_team_model'),\n    ]\n\n    operations = [\n        migrations.AlterField(\n            model_name='team',\n            name='email',\n            field=models.EmailField(blank=True, max_length=70, null=True),\n        ),\n        migrations.AlterField(\n            model_name='team',\n            name='github_url',\n            field=models.CharField(blank=True, max_length=200, null=True),\n        ),\n        migrations.AlterField(\n            model_name='team',\n            name='linkedin_url',\n            field=models.CharField(blank=True, max_length=200, null=True),\n        ),\n        migrations.AlterField(\n            model_name='team',\n            name='personal_website',\n            field=models.CharField(blank=True, max_length=200, null=True),\n        ),\n    ]\n", "path": "apps/web/migrations/0004_change_team_model_field_type.py"}]}