Don't return images if a tag does not have a legacy image

endpoints/api/tag.py

@@ -227,6 +227,9 @@ class RepositoryTagImages(RepositoryParamResource):
     if tag_ref is None:
       raise NotFound()
 
+    if tag_ref.legacy_image_if_present is None:
+      return {'images': []}
+
     image_id = tag_ref.legacy_image.docker_image_id
 
     all_images = None