title: Django OneToOneField default value
body: <p>Recently I added an OneToOneField in django model , I set None as default value in this field . then i got this error :</p>
  
  <p><strong><code>django.db.utils.IntegrityError: NOT NULL constraint failed: user_myuser.album_id</code></strong></p>
  
  <p>model : </p>
  
  <pre><code>class MyUser(models.Model):
  
      username = models.CharField(unique=True, max_length=25)
      first_name = models.CharField(max_length=50, default='')
      last_name = models.CharField(max_length=70, default='')
      register_date = models.DateTimeField(default=timezone.now)
      update_at = models.DateTimeField(auto_now=True)
  
      # the default value problem is here 
      album = models.OneToOneField(Album, auto_created=True, 
      on_delete=models.CASCADE, default=None)
  </code></pre>
  
  <p>And Here is Album Model : </p>
  
  <pre><code>class Album(models.Model):
      name = models.CharField(max_length=100, null=True)
      author = models.CharField(max_length=100, null=True)
      picture_address = models.ImageField(upload_to=album_upload_destination, null=True)
      creation_year = models.IntegerField(default=-1)
      rate = models.IntegerField(default=0)
      timestamp = models.DateTimeField(auto_now_add=True)
  </code></pre>