fix #308064: assertion failure on empty nashville number

Resolves: https://musescore.org/en/node/308064

Playback of Nashville numbers was not handling empty elements
(for example, when hitting space without typing anything).
Fixed to check for this and skip realization.

libmscore/harmony.cpp

@@ -1251,6 +1251,7 @@ const RealizedHarmony& Harmony::getRealizedHarmony()
             offset = interval.chromatic;
 
       //Adjust for Nashville Notation, might be temporary
+      // TODO: set dirty on add/remove of keysig
       if (_harmonyType == HarmonyType::NASHVILLE && !_realizedHarmony.valid()) {
             Key key = staff()->key(tick());
             //parse root

libmscore/realizedharmony.cpp

@@ -215,7 +215,8 @@ void RealizedHarmony::update(int rootTpc, int bassTpc, int transposeOffset /*= 0
             return;
             }
 
-      _notes = generateNotes(rootTpc, bassTpc, _literal, _voicing, transposeOffset);
+      if (tpcIsValid(rootTpc))
+            _notes = generateNotes(rootTpc, bassTpc, _literal, _voicing, transposeOffset);
       _dirty = false;
       }