Let serializers have empty field lists (#251)
Let serializers have empty field lists (#251)

Currently, `fields = ()` in a serializer doesn't work because `get_serializer_fields` returns `None`
django-json-api/django-rest-framework-json-api,pombredanne/django-rest-framework-json-api,django-json-api/django-rest-framework-json-api,leo-naeka/django-rest-framework-json-api,Instawork/django-rest-framework-json-api,django-json-api/rest_framework_ember,abdulhaq-e/django-rest-framework-json-api