title: Django JSON field in Serializer validated_data is None
body: <p>I have a Django Model which looks something like this:</p>
  
  <pre><code>class Foo:
    data = JSONField(encoder=DjangoJSONEncoder, default=dict)
  </code></pre>
  
  <p>And I have the corresponding serializer for it which is pretty basic:</p>
  
  <pre><code>class FooSerializer(BaseSerializer):
  
    class Meta:
      model = models.Foo
      fields = '__all__'
  </code></pre>
  
  <p>What I want to do is to have some validations for that JSON field in the serializer. I tried doing this via overriding the <code>validate</code> and <code>create</code> functions but in both cases the value for <code>attrs.data</code> or <code>validated_data.data</code> is <code>None</code>. </p>
  
  <p>The weird part is that if I create the same validations in the <code>pre-save</code> or <code>post-save</code> signals then the <code>instance.data</code> value is not <code>None</code> and actually has the value I'm passing in the request. </p>
  
  <p>I'm wondering if I'm doing something wrong or does DRF not support doing validations for JSON fields in the serializer and just expect you to do it in the signals. That seems kinda wrong. </p>