title: Exclude a returned representation in django restframework
body: <p>I have a Serializer but I want to set a calculation whether to include it in serialized return objects or not.</p>
  <pre><code>class TradeSerializer(serializers.ModelSerializer):
      .
      .
      .
      .
      def to_representation(self, instance):
          ret = super().to_representation(instance)
          if ret['countdown'] &lt; 5:
              # STOP! Do not return an object anymore!
              return
          return ret
  </code></pre>
  <p>But the syntax above will return something like <code>ret[None]</code>. I looking for an absolute exclusion</p>