repos: Prevent individual cloud default external service syncing

Created by: tsenart

This commit makes it so we don't enqueue external service sync jobs for cloud default external services also when there are no rows in the external_service_sync_jobs table.

This case was unfortunately missed in #24412, which was a follow up from a very similar incident to the one we are now handling, so it did not prevent it from happening again :sadpanda:

Additionally, we prevent any cloud default service from syncing in the Syncer.SyncExternalService method by early exiting with an error (defense in depth).