[dataengines/hotplug] Don't crash when there is no default file manager

Nicolas Fellarequested to merge
work/nico/fm-crash into master

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

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