[KDEPlatformFileDialog] Don't do stat if baseUrl didn't change

Qt might call `setDirectory` twice on dialog initialization, the second
one coming from `restoreFromSettings`.
This would needlessly trigger another `StatJob` if the URL didn't change.


(cherry picked from commit fffe10df)
2 jobs for Plasma/5.24 in 2 minutes and 13 seconds (queued for 46 seconds)