Drop bits of relations_slug tests which don't mirror existing tests.

rest_framework/relations.py

@@ -105,6 +105,9 @@ class RelatedField(WritableField):
             value = getattr(obj, self.source or field_name)
         except ObjectDoesNotExist:
             return None
+
+        if value is None:
+            return None
         return self.to_native(value)
 
     def field_from_native(self, data, files, field_name, into):

rest_framework/tests/relations_hyperlink.py

@@ -3,6 +3,7 @@ from rest_framework import serializers
 from rest_framework.compat import patterns, url
 from rest_framework.tests.models import ManyToManyTarget, ManyToManySource, ForeignKeyTarget, ForeignKeySource, NullableForeignKeySource, OneToOneTarget, NullableOneToOneSource
 
+
 def dummy_view(request, pk):
     pass
 
@@ -16,6 +17,7 @@ urlpatterns = patterns('',
     url(r'^nullableonetoonesource/(?P<pk>[0-9]+)/$', dummy_view, name='nullableonetoonesource-detail'),
 )
 
+
 class ManyToManyTargetSerializer(serializers.HyperlinkedModelSerializer):
     sources = serializers.ManyHyperlinkedRelatedField(view_name='manytomanysource-detail')