fix: report zero volume from useTrackVolume

[RaphaelFakhri]

Fixes useTrackVolume so it reports silence.

The native volume processors emit volume: 0 when a track is silent or muted. The event handler checked event.volume for truthiness, so a value of 0 was ignored and the hook kept returning the last non-zero volume. A level meter built on the hook stays stuck at its last level after the speaker stops.

The handler now only skips events where volume is null or undefined.

Testing:

• The repo has no jest tests for hooks yet, so this PR keeps the change to the one-line fix and a changeset.
• A local jest test that emits a non-zero volume followed by 0 fails before the change (Expected: 0, Received: 0.4) and passes after it. It needs react-test-renderer and a jest environment change, so it is not included. It can be added on request.
• yarn lint and yarn typescript pass.

[changeset-bot]

🦋 Changeset detected

Latest commit: b3bc601

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@livekit/react-native	Patch

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.