[Containment Appearance] Make sure to always load a config view

If StackView is empty, currentItem will be null and we would break all over the place.
This can happen when you have a wallpaper without config, so "sourceFile" would stay empty
and thus onSourceFileChanged wouldn't be triggered and we wouldn't load anything into our StackView.

BUG: 360862
FIXED-IN: 5.8.6

Differential Revision: https://phabricator.kde.org/D3911