Sets lastUpdatedBy to null when it is empty on github sync update (#18788)

GitOrigin-RevId: bce67f7d3dbc59facc626cc63af94df46afd8f24

services/document-updater/app/js/DocumentManager.js

@@ -222,7 +222,7 @@ const DocumentManager = {
       version,
       ranges,
       lastUpdatedAt,
-      lastUpdatedBy
+      lastUpdatedBy || null
     )
     await RedisManager.promises.clearUnflushedTime(docId)
     return result