fix: skip undefined tune data when a tune is in its default state

[Hyhyhyyy]

Summary

Closes #1683.

When a Block Tune's save() returns undefined (i.e. the tune is in its default state and has nothing to persist), the value was still written to tunesData[name]. This leaves an explicit undefined entry in the saved tunes object, which then gets carried through load/save cycles.

This guards the assignment so that only defined tune data is stored:

const tuneData = tune.save() as BlockTuneData | undefined;

if (tuneData !== undefined) {
  tunesData[name] = tuneData;
}

Changes

• src/components/block/index.ts: skip storing a tune's data when save() returns undefined.

Notes

The BlockTune.save() type is BlockTuneData, but in practice a tune may intentionally return undefined to signal "no data / default state" (as described in the issue). The local as BlockTuneData | undefined cast keeps the type checker happy while allowing the runtime guard.

How to test

1. Create a block tune whose save() returns undefined in its default state.
2. Save the block and inspect the output JSON — the tune key should no longer appear with an undefined value.