Add a handler for broken tags in the tag backfill system

This will generate a tag pointing to an empty manifest; the tag will be broken, but as it is *already* broken, at least the backfill can complete

data/registry_model/registry_pre_oci_model.py

@@ -478,6 +478,9 @@ class PreOCIModel(SharedModel, RegistryDataInterface):
 
     # Write the manifest to the DB.
     manifest = self._build_manifest_for_legacy_image(tag_obj.name, tag_obj.image)
+    if manifest is None:
+      return None
+
     blob_query = model.storage.lookup_repo_storages_by_content_checksum(repo,
                                                                         manifest.checksums)