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!)
           2. Start a puzzle, quit palapeli
           3. Start palapeli and proceed with the last puzzle
              -> puzzle is loaded correctly
           4. 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