title: Filter Queryset on empty ImageField
body: <p>I have the following class:</p>
  
  <pre><code>class Book(models.Model):
      picture = models.ImageField(upload_to='books/', blank=True, null=True)
      ...
  </code></pre>
  
  <p>I now want to filter the books without a picture. I tried the following:</p>
  
  <pre><code>Book.objects.filter(picture__isnull=True)
  </code></pre>
  
  <p>The problem is, that the picture is an empty varchar ('') in the db and not null. What to do?</p>