revisions: convert empty fieldValue to None

We check to see if the query was empty and return `None`,
however the query will be non-empy but then `fieldValue`
will be empty, which fails when submitting to BigQuery.

stats.py

@@ -148,7 +148,7 @@ def get_bug_id(revision: DiffDb.Revision, bug_id_query) -> Optional[int]:
     if not bug_id_query_result:
         return None
 
-    return bug_id_query_result.fieldValue
+    return bug_id_query_result.fieldValue or None
 
 
 class PhabricatorEdgeConstant(IntEnum):