title: Django-DB-Migrations: cannot ALTER TABLE because it has pending trigger events
body: <p>I want to remove null=True from a TextField:</p>
  
  <pre><code>-    footer=models.TextField(null=True, blank=True)
  +    footer=models.TextField(blank=True, default='')
  </code></pre>
  
  <p>I created a schema migration:</p>
  
  <pre><code>manage.py schemamigration fooapp --auto
  </code></pre>
  
  <p>Since some footer columns contain <code>NULL</code> I get this <code>error</code> if I run the migration:</p>
  
  <blockquote>
    <p>django.db.utils.IntegrityError: column "footer" contains null values</p>
  </blockquote>
  
  <p>I added this to the schema migration:</p>
  
  <pre><code>    for sender in orm['fooapp.EmailSender'].objects.filter(footer=None):
          sender.footer=''
          sender.save()
  </code></pre>
  
  <p>Now I get:</p>
  
  <pre><code>django.db.utils.DatabaseError: cannot ALTER TABLE "fooapp_emailsender" because it has pending trigger events
  </code></pre>
  
  <p>What is wrong?</p>