Fix gmail auth to return empty string instead of response object on e…

…rror

src/backend/tools/gmail/auth.py

@@ -82,7 +82,7 @@ def retrieve_auth_token(
             logger.error(
                 event=f"[Gmail] Error retrieving auth token: {response_body}"
             )
-            return response
+            return ""
 
         tool_auth_crud.create_tool_auth(
             session,