pluginloader: never return nullptr on loadApplet

Harald Sitterrequested to merge
work/sitter/fakeapplet into master

we always want to leave a user visible trace of the applet even when it fails to load. so make sure to synthesize useful fake applets in that case

this should prevent crashes where the applet is defunct for whatever reason (e.g. lack of rebuild for soname change) and allow for assertions about the validity of applets to pass

Fixes PLASMA-WORKSPACE-4DJK

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