kcms/mouse: Don't crash when no supported backend is found
If InputBackend::implementation() and ConfigPlugin::implementation() can return a nullptr, we need to have null pointer checks where they get used.
Also, remove some dead code without these checks to avoid confusion.
This will leave the KCM completely empty in case the libinput backend isn't available. Same behavior as kcm_touchpad. That's not optimal, but is an easy fix without introducing new translatable strings or functionality. For 6.1, I'm looking at getting a proper QML error message into the settings page in addition to some other changes (see #117).
Edited by Jakob Petsovits