Add missing method

workers/tagbackfillworker.py

@@ -102,6 +102,10 @@ class BrokenManifest(ManifestInterface):
   def get_requires_empty_layer_blob(self, content_retriever):
     return False
 
+  def convert_manifest(self, allowed_mediatypes, namespace_name, repo_name, tag_name,
+                       content_retriever):
+    return None
+
 
 class TagBackfillWorker(Worker):
   def __init__(self):