title: Allow empty list in serializers.ListField
body: <p>I am using Django REST framework 3.3 and am trying to serialize a list that could be empty with the provided serializers.ListField class that is included in the framework. My current instantiation of the field looks like this </p>
  
  <pre><code> countrypreferences = serializers.ListField(child=serializers.IntegerField(),
                                              source='country_preference_ids',
                                              allow_null=True)
  </code></pre>
  
  <p>When testing the API I always seem to get a 400 response if I let the field be an empty list. It would seem like this kind of functionality would be pretty common but I can't find a way to allow the empty list. Thanks!</p>