Fix empty viewset compatibility w/ browsable API (#263)

* Add rendering to 'disabled' viewset test
* Allow views with empty filterset_class

rest_framework_filters/backends.py

@@ -29,6 +29,10 @@ def patch_for_rendering(self, request):
         def get_filterset_class(view, queryset=None):
             filterset_class = original(view, queryset)
 
+            # Don't break if filterset_class is not provided
+            if filterset_class is None:
+                return None
+
             # django-filter compatibility
             if issubclass(filterset_class, FilterSet):
                 filterset_class = filterset_class.disable_subset(depth=1)

tests/test_backends.py

@@ -106,6 +106,11 @@ def test_disabled(self):
         # filterset should be None, method should not error
         self.assertIsNone(backend().get_filterset(request, view.queryset, view))
 
+        # patched method should not error
+        backend = backend()
+        with backend.patch_for_rendering(request):
+            self.assertIsNone(backend.get_filterset(request, view.queryset, view))
+
 
 class BackendRenderingTests(APITestCase):