Optimize lookup of shared global blobs

Currently, we only have one (the shared empty layer), but this should make the blob lookups for repositories significantly faster, as we won't need to do the massive join.

data/model/storage.py

@@ -264,6 +264,9 @@ def get_layer_path_for_storage(storage_uuid, cas_path, content_checksum):
 def lookup_repo_storages_by_content_checksum(repo, checksums, by_manifest=False):
   """ Looks up repository storages (without placements) matching the given repository
       and checksum. """
+  if not checksums:
+    return []
+
   # There may be many duplicates of the checksums, so for performance reasons we are going
   # to use a union to select just one storage with each checksum
   queries = []