title: AttributeError: 'GenericForeignKey' object has no attribute 'null'
body: <p>I'm getting a very weird error when I try any method (delete, save, etc) on a model that holds a Generic Foreign Key. My model can hold different type of card types, and a job to process them:</p>
  
  <pre><code>class JobCards(models.Model):
  
      class Meta:
          unique_together = ('content_type', 'object_id')
  
      content_type = models.ForeignKey(
          ContentType,
          on_delete=models.CASCADE,
          help_text=_('Card type')
      )
      object_id = models.PositiveIntegerField(
          help_text=_('Card id')
      )
      content_object = GenericForeignKey()
  
      job = models.ForeignKey(
         Job,
         on_delete=models.CASCADE
      )
  </code></pre>
  
  <p>So operations like:</p>
  
  <pre><code>JobCards.objects.create(content_object=card, job=job)
  </code></pre>
  
  <p>or</p>
  
  <pre><code>job_card = JobCards.objects.get(
                job=job,
                content_type=ContentType.objects.get_for_model(card),
                object_id=card.id
             )
  job_card.delete()
  </code></pre>
  
  <p>will fail with such error.</p>
  
  <p>I should mention that I put two GenericRelation in the card models, but even removing them doesn't change the end result:</p>
  
  <pre><code>job_cards = GenericRelation(JobCards, related_query_name='card')
  </code></pre>