Resolve unsoundness caught by pytype --strict-none-binding.

PiperOrigin-RevId: 555183577

Author: tf-transform-team

tensorflow_transform/beam/cached_impl_test.py

@@ -503,6 +503,8 @@ def _publish_rendered_dot_graph_file(self,
                                                  dataset_keys, pcoll_cache_dict)
     def sort_value_node_values(cache_dict):
       result = []
+      if cache_dict is None:
+        return result
       for dataset_cache in cache_dict.values():
         result.extend(dataset_cache.values())
       return sorted(result, key=str)