diff --git a/ecs_deploy/slack.py b/ecs_deploy/slack.py
index 2e5761c..fa252cc 100644
--- a/ecs_deploy/slack.py
+++ b/ecs_deploy/slack.py
@@ -60,7 +60,7 @@ class SlackNotification(object):
             messages.append(('Comment', comment))
 
         for diff in task_definition.diff:
-            if diff.field == 'image' and diff.value.endswith(':' + tag):
+            if tag and diff.field == 'image' and diff.value.endswith(':' + tag):
                 continue
             if diff.field == 'environment':
                 messages.append(('Environment', '_sensitive (therefore hidden)_'))

Slack notifications don't work when deploying new image
Hi! Thanks for the tool.

I was playing around with the Slack notifications and they work fine when using the -t flag but not when passing images with the -i one.

Looks like tag variable is null over here:
 https://github.com/fabfuel/ecs-deploy/blob/1116ba7d9f7dd256b9668bbcf9cad973da4fde5d/ecs_deploy/cli.py#L87
 
Here's the full stack trace:
```
Traceback (most recent call last):
  File "/usr/local/bin/ecs", line 8, in <module>
    sys.exit(ecs())
  File "/usr/local/lib/python3.7/site-packages/click/core.py", line 722, in __call__
    return self.main(*args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/click/core.py", line 697, in main
    rv = self.invoke(ctx)
  File "/usr/local/lib/python3.7/site-packages/click/core.py", line 1066, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/local/lib/python3.7/site-packages/click/core.py", line 895, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/usr/local/lib/python3.7/site-packages/click/core.py", line 535, in invoke
    return callback(*args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/ecs_deploy/cli.py", line 87, in deploy
    slack.notify_start(cluster, tag, td, comment, user, service=service)
  File "/usr/local/lib/python3.7/site-packages/ecs_deploy/slack.py", line 63, in notify_start
    if diff.field == 'image' and diff.value.endswith(':' + tag):
TypeError: can only concatenate str (not "NoneType") to str
```
diff --git a/tests/test_slack.py b/tests/test_slack.py
index d9b0489..0134882 100644
--- a/tests/test_slack.py
+++ b/tests/test_slack.py
@@ -120,6 +120,39 @@ def test_notify_start(post_mock, url, service_match, task_definition):
     post_mock.assert_called_with(url, json=payload)
 
 
+@patch('requests.post')
+def test_notify_start_without_tag(post_mock, url, service_match, task_definition):
+    post_mock.return_value = NotifyResponseSuccessfulMock()
+
+    task_definition.set_images(webserver=u'new-image:my-tag', application=u'app-image:another-tag')
+    task_definition.set_environment((('webserver', 'foo', 'baz'),))
+
+    slack = SlackNotification(url, service_match)
+    slack.notify_start('my-cluster', None, task_definition, 'my-comment', 'my-user', 'my-service', 'my-rule')
+
+    payload = {
+        'username': 'ECS Deploy',
+        'attachments': [
+            {
+                'pretext': 'Deployment has started',
+                'color': None,
+                'fields': [
+                    {'title': 'Cluster', 'value': 'my-cluster', 'short': True},
+                    {'title': 'Service', 'value': 'my-service', 'short': True},
+                    {'title': 'Scheduled Task', 'value': 'my-rule', 'short': True},
+                    {'title': 'User', 'value': 'my-user', 'short': True},
+                    {'title': 'Comment', 'value': 'my-comment', 'short': True},
+                    {'title': 'image', 'value': 'new-image:my-tag', 'short': True},
+                    {'title': 'image', 'value': 'app-image:another-tag', 'short': True},
+                    {'title': 'Environment', 'value': '_sensitive (therefore hidden)_', 'short': True}
+                ]
+            }
+        ]
+    }
+
+    post_mock.assert_called_with(url, json=payload)
+
+
 @patch('requests.post')
 @freeze_time()
 def test_notify_success(post_mock, url, service_match, task_definition):

pip install -e .[dev]
parse_log_pytest
requirements.txt
pytest pytest-cov pytest-mock mock requests boto3 freezegun
apt-get update apt-get install -y gcc
3.6
pytest --no-header -rA --tb=line --color=no -p no:cacheprovider -W ignore::DeprecationWarning
attrs==22.2.0
boto3==1.23.10
botocore==1.26.10
certifi==2021.5.30
charset-normalizer==2.0.12
click==6.7
coverage==6.2
dictdiffer==0.8.0
-e git+https://github.com/fabfuel/ecs-deploy.git@1116ba7d9f7dd256b9668bbcf9cad973da4fde5d#egg=ecs_deploy
freezegun==1.2.2
future==1.0.0
idna==3.10
importlib-metadata==4.8.3
iniconfig==1.1.1
jmespath==0.10.0
mock==5.2.0
packaging==21.3
pluggy==1.0.0
py==1.11.0
pyparsing==3.1.4
pytest==7.0.1
pytest-cov==4.0.0
pytest-mock==3.6.1
python-dateutil==2.9.0.post0
requests==2.27.1
s3transfer==0.5.2
six==1.17.0
tomli==1.2.3
typing_extensions==4.1.1
urllib3==1.26.20
zipp==3.6.0

name: ecs-deploy
channels:
  - defaults
  - https://repo.anaconda.com/pkgs/main
  - https://repo.anaconda.com/pkgs/r
  - conda-forge
dependencies:
  - _libgcc_mutex=0.1=main
  - _openmp_mutex=5.1=1_gnu
  - ca-certificates=2025.2.25=h06a4308_0
  - certifi=2021.5.30=py36h06a4308_0
  - ld_impl_linux-64=2.40=h12ee557_0
  - libffi=3.3=he6710b0_2
  - libgcc-ng=11.2.0=h1234567_1
  - libgomp=11.2.0=h1234567_1
  - libstdcxx-ng=11.2.0=h1234567_1
  - ncurses=6.4=h6a678d5_0
  - openssl=1.1.1w=h7f8727e_0
  - pip=21.2.2=py36h06a4308_0
  - python=3.6.13=h12debd9_1
  - readline=8.2=h5eee18b_0
  - setuptools=58.0.4=py36h06a4308_0
  - sqlite=3.45.3=h5eee18b_0
  - tk=8.6.14=h39e8969_0
  - wheel=0.37.1=pyhd3eb1b0_0
  - xz=5.6.4=h5eee18b_1
  - zlib=1.2.13=h5eee18b_1
  - pip:
      - attrs==22.2.0
      - boto3==1.23.10
      - botocore==1.26.10
      - charset-normalizer==2.0.12
      - click==6.7
      - coverage==6.2
      - dictdiffer==0.8.0
      - freezegun==1.2.2
      - future==1.0.0
      - idna==3.10
      - importlib-metadata==4.8.3
      - iniconfig==1.1.1
      - jmespath==0.10.0
      - mock==5.2.0
      - packaging==21.3
      - pluggy==1.0.0
      - py==1.11.0
      - pyparsing==3.1.4
      - pytest==7.0.1
      - pytest-cov==4.0.0
      - pytest-mock==3.6.1
      - python-dateutil==2.9.0.post0
      - requests==2.27.1
      - s3transfer==0.5.2
      - six==1.17.0
      - tomli==1.2.3
      - typing-extensions==4.1.1
      - urllib3==1.26.20
      - zipp==3.6.0
prefix: /opt/conda/envs/ecs-deploy

tests/test_slack.py::test_notify_start_without_tag
tests/test_slack.py::test_get_payload_without_messages tests/test_slack.py::test_get_payload_with_messages tests/test_slack.py::test_notify_start_without_url tests/test_slack.py::test_notify_start tests/test_slack.py::test_notify_success tests/test_slack.py::test_notify_success_without_url tests/test_slack.py::test_notify_failure_without_url tests/test_slack.py::test_notify_start_failed tests/test_slack.py::test_notify_success_failed tests/test_slack.py::test_notify_failure_failed
ecs_deploy/slack.py
ecs_deploy/slack.py