Fix for QStandardPaths porting

Summary:
Since QStandardPaths::locate() only returns a valid filename when
the file exists, we have to make sure to not pass an empty
filename to KConfig when trying to restore / save a puzzle

Test Plan:

1. remove ~/.local/share/palapeli (very important!)
   1. Start a puzzle, quit palapeli
   2. Start palapeli and proceed with the last puzzle -> puzzle is loaded correctly
   3. Try to restart puuzle (after finished or within) -> all pieces are placed randomly

Reviewers: KDE Games, ltoscano

Reviewed By: ltoscano

Subscribers: ltoscano, KDE Games

Differential Revision: https://phabricator.kde.org/D7502