Set default app data dir if blank
Set default app data dir if blank

Normally the setup wizard would set this up.  But if the user
deliberately skipped the setup, "app/app_data" won't be in the settings,
and then all sorts of things (including the DEBUG messages) will be broken.

Tarsnap/tarsnap-gui,Tarsnap/tarsnap-gui,Tarsnap/tarsnap-gui,Tarsnap/tarsnap-gui,Tarsnap/tarsnap-gui