Fix NPE in changing the expiration of a tag

data/model/tag.py

@@ -895,7 +895,7 @@ def set_tag_end_ts(tag, end_ts):
   """ Sets the end timestamp for a tag. Should only be called by change_tag_expiration
       or tests.
   """
-  end_ms = end_ts * 1000
+  end_ms = end_ts * 1000 if end_ts is not None else None
 
   with db_transaction():
     # Note: We check not just the ID of the tag but also its lifetime_end_ts, to ensure that it has