If nothing is restored, it should not record as a task in mysql.tidb_restore_registry

## Enhancement

Currently, even when a restore job ends up restoring nothing (i.e., it’s an empty restore), we still record a task entry in mysql.tidb_restore_registry. This can lead to unnecessary metadata entries and may cause potential conflicts with future restore operations.

However, since we can detect that this is an empty restore, it is more appropriate to skip task registration altogether in such cases to avoid polluting the registry and reduce the chance of false-positive conflicts.