Crash on entry of empty Nashville number
Introduced by the chord symbol playback facility, we don't handle empty Nashville numbers correctly.
1) click any note or rest in any score with chord symbol playback enabled
2) Add / Text / Nashville Number
3) enter "1"
4) press space, verify you hear the playback
5) press space again - crash as we try to play the empty chord
FIx seems simple, only generate notes is rootTpc is valid.