More robust handling of the tuning dictionary file

[cameronwhite]

If the tunings.json file in the prefs folder ends up being empty for some reason (e.g. the default tuning dictionary from the install folder wasn't found), this ends up causing a perpetual crash on exit
We should do something better here: e.g. avoid trying to save it when empty, and/or log a warning instead of popping up a crash dialog.