fix: strip null synth_engine from project JSON

- Frontend serialize: use undefined instead of null for synth_engine
- Backend validate: strip None synth_engine from tracks before schema validation
- Prevents 'None is not of type object' schema error

app/static/js/app.jsx

@@ -6137,7 +6137,7 @@ const serializeTracksList = (tracksList, secondsPerBar) => {
       soundfont_id: t.soundfont_id || (t.synth_engine ? t.synth_engine.soundfont_id : null),
       soundfont_bank: t.soundfont_bank !== undefined ? t.soundfont_bank : (t.synth_engine ? t.synth_engine.soundfont_bank : null),
       soundfont_program: t.soundfont_program !== undefined ? t.soundfont_program : (t.synth_engine ? t.synth_engine.soundfont_program : null),
-      synth_engine: t.synth_engine || null,
+      synth_engine: t.synth_engine || undefined,
       items: items
     };
   });
@@ -6209,7 +6209,7 @@ const deserializeTracksList = (schemaTracks, secondsPerBar, sectionStore) => {
       soundfont_id: t.soundfont_id || null,
       soundfont_bank: t.soundfont_bank !== null ? t.soundfont_bank : undefined,
       soundfont_program: t.soundfont_program !== null ? t.soundfont_program : undefined,
-      synth_engine: t.synth_engine || null
+      synth_engine: t.synth_engine || undefined
     };
   });
 };