Fix assertion while running testShutdownWithRunningJobs

Summary:
Don't assert the validity of the IndexedString (while calling toUrl
on it), in the JobPlan's updateReady callback while the core is
shutting down. This is potentially getting called when the database
has been shutdown, leaving an empty/invalid string behind.

Subscribers: kdevelop-devel

Differential Revision: https://phabricator.kde.org/D8904