appDisplay: Consider admin defined folders before creating default ones

appDisplay: Consider admin defined folders before creating default ones

The system administrator might have defined some default folders system
wide via dconf default values, so checking for user defined values is
not enough. This restores the condition to also skip creating default
ones if the value is not empty. It still skips the creation of default
folders if the empty value was the result of a user action to allow
users deleting all folders.

Fixes: b0d396c82 ("appDisplay: Only create default folders on the first start")
Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/9062
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/4087>