[dataengines/hotplug] Don't crash when there is no default file manager
Reason for the change
preferredService may return null, and does according to crash reports
Test plan
None
Screenshots or screen recordings
Bugs fixed
https://crash-reports.kde.org/organizations/kde/issues/3509
https://crash-reports.kde.org/organizations/kde/issues/55847
Edited by Nicolas Fella