title: "This field is required" when GenericRelation field is marked as not required in serializer
body: <p>I have a model set up with a GenericRelation named <code>tags</code>. The GenericRelation object defaults the <code>blank</code> kwargs attribute to True - so null is allowed at the Model Level:</p>
  
  <pre><code>class Character(models.Model):
      name = models.CharField(max_length=200)
      date_of_birth = models.DateField()
      tags = GenericRelation(TaggableTag, content_type_field='taggable_type', object_id_field='taggable_id')
  </code></pre>
  
  <p>Within the serializer, I have attached a serializer to this tags field to ensure it's eager loaded on each request for Character information.</p>
  
  <pre><code>class CharacterSerializer(serializers.HyperlinkedModelSerializer):
      tags = TaggableTagSerializer(many=True)
  
      class Meta:
          model = Character
          fields = ('id', 'name', 'date_of_birth', 'tags')
          extra_kwargs = {
              'tags': {'required': False, 'allow_null': True, 'default': None}
          }
          validators = []
  </code></pre>
  
  <p>As you can see, I've gone bit overkill on the validation rules, but all to just be doubly sure that it doesn't force the 'tags' field to be required.</p>
  
  <p>However, the POST route seems to still return the following validation message:</p>
  
  <pre><code>HTTP 400 Bad Request
  Allow: GET, POST, HEAD, OPTIONS
  Content-Type: application/json
  Vary: Accept
  
  {
      "tags": [
          "This field is required."
      ]
  }
  </code></pre>
  
  <p>I'm unsure what to do here, I've followed the documentation on the Django Rest Framework <a href="https://www.django-rest-framework.org/api-guide/validators/#optional-fields" rel="nofollow noreferrer">page about Validators</a> - I'm certain this might be because it's a GenericRelation field rather than a normal field, but I'm unsure how to solve it. Can you help?</p>