Add failing test for fallback to pk-only optimization

Not sure if this is the right place to put the test or where the situation comes up in the real world when not using ORM hacks (like how we found it), but it *does* test the thing.

tests/test_relations.py

@@ -3,7 +3,7 @@ import uuid
 import pytest
 from _pytest.monkeypatch import MonkeyPatch
 from django.conf.urls import url
-from django.core.exceptions import ImproperlyConfigured
+from django.core.exceptions import ImproperlyConfigured, ObjectDoesNotExist
 from django.test import override_settings
 from django.utils.datastructures import MultiValueDict
 
@@ -167,6 +167,22 @@ class TestHyperlinkedRelatedField(APISimpleTestCase):
         representation = self.field.to_representation(MockObject(pk=''))
         assert representation is None
 
+    def test_serialize_empty_relationship_attribute(self):
+        class TestSerializer(serializers.Serializer):
+            via_unreachable = serializers.HyperlinkedRelatedField(
+                source='does_not_exist.unreachable',
+                view_name='example',
+                read_only=True,
+            )
+
+        class TestSerializable:
+            @property
+            def does_not_exist(self):
+                raise ObjectDoesNotExist
+
+        serializer = TestSerializer(TestSerializable())
+        assert serializer.data == {'via_unreachable': None}
+
     def test_hyperlinked_related_lookup_exists(self):
         instance = self.field.to_internal_value('http://example.org/example/foobar/')
         assert instance is self.queryset.items[0]