Skip empty sound sources during sound autodetection

[Copilot]

Autodetecting sounds from placed sound sources was treating unassigned sound sources as real sound IDs and adding the internal sentinel value -1 to SelectedSounds. This caused the sound list UI to surface a bogus -1 entry when empty sound sources existed in the map.

• Behavior change

  • AssignSoundSourcesSounds now ignores empty SoundSourceInstances instead of copying their sentinel ID into level sound selection.

• Why this is the correct boundary

  • Empty sound sources are already modeled explicitly via SoundSourceInstance.IsEmpty.
  • Other code paths already treat -1 as "no sound assigned", so autodetection now follows the same contract instead of leaking the sentinel into user-facing selection state.

• Scope

  • The change is isolated to sound-source-based autodetection.
  • Assigned sound sources continue to contribute their real sound IDs unchanged.

var soundSource = instance as SoundSourceInstance;
if (soundSource.IsEmpty)
    continue;

if (!settings.SelectedSounds.Contains(soundSource.SoundId))
    settings.SelectedSounds.Add(soundSource.SoundId);