          # TODO: filter queryset using:
          # .using(db).complex_filter(self.rel.limit_choices_to)
          rel = model_field.rel.to
          kwargs = {
              'blank': model_field.blank,
              'queryset': rel._default_manager,
              'view_name': self._get_default_view_name(rel)
          }
          if to_many:
              return ManyHyperlinkedRelatedField(**kwargs)
 
</s> remove             'blank': model_field.blank,
 </s> add             'null': model_field.null, </s> remove         if value in (None, '') and not self.blank:
 </s> add         if value in (None, '') and not self.null: </s> remove         elif value in (None, '') and self.blank:
 </s> add         elif value in (None, '') and self.null:
            'null': model_field.null,
          # TODO: filter queryset using:
          # .using(db).complex_filter(self.rel.limit_choices_to)
          rel = model_field.rel.to
          kwargs = {
            'null': model_field.null,
              'queryset': rel._default_manager,
              'view_name': self._get_default_view_name(rel)
          }
          if to_many:
              return ManyHyperlinkedRelatedField(**kwargs)
keep keep keep keep replace keep keep keep keep keep
 Add null related field option 
rest_framework/serializers.py