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)
 

data/registry_model/shared.py

@@ -418,7 +418,11 @@ class SharedModel:
     repo_name = repo.name
 
     # Find the v1 metadata for this image and its parents.
-    parents = model.image.get_parent_images(namespace_name, repo_name, legacy_image_row)
+    try:
+      parents = model.image.get_parent_images(namespace_name, repo_name, legacy_image_row)
+    except model.DataModelException:
+      logger.exception('Could not load parent images for legacy image %s', legacy_image_row.id)
+      return None
 
     # If the manifest is being generated under the library namespace, then we make its namespace
     # empty.