title: Remove null fields from Django Rest Framework response
body: <p>I have developed an API using django-rest-framework.
  I am using ModelSerializer to return data of a model.</p>
  
  <p><strong>models.py</strong></p>
  
  <pre><code>class MetaTags(models.Model):
      title = models.CharField(_('Title'), max_length=255, blank=True, null=True)
      name = models.CharField(_('Name'), max_length=255, blank=True, null=True)
  </code></pre>
  
  <p><strong>serializer.py</strong></p>
  
  <pre><code>class MetaTagsSerializer(serializers.ModelSerializer):
      class Meta:
          model = MetaTags
  </code></pre>
  
  <p><strong>response</strong></p>
  
  <pre><code>{
      "meta": {
          "title": null,
          "name": "XYZ"
      }
  }
  </code></pre>
  
  <p>Ideally in an API response any value which is not present should not be sent in the response.
  When the <code>title</code> is <code>null</code> I want the response to be:</p>
  
  <pre><code>{
      "meta": {
          "name": "XYZ"
      }
  }
  </code></pre>