Just look whether we have a new service instead of assuming the old one is empty.
This makes atomic service owner change when the owner is replaced work.

Started a copy, killed plasmashell, restarted it, got a plasma job tracker (i.e. still works)
Had a separate job progress viewer running, Started a copy, started plasmashell started, it claimed the service and took it away from the other, now I got the job re-show in plasmashell