fix: Subtitle off and subtitle stream selection logic

[schembriaiden]

Pull Request Description

This pull request improves the handling of default subtitle and audio stream selection in the playback models and ensures more robust and consistent behavior when no valid stream is found. The main focus is on returning a default "no stream" object instead of null, and updating the player wrappers to correctly handle these cases.

Improvements to stream selection logic:

• Updated the PlaybackModelExtension in playback_model.dart so that defaultSubStream and defaultAudioStream return a default "no stream" object (e.g., SubStreamModel.no(), AudioStreamModel.no()) when no valid stream is found, instead of returning null. This makes downstream code safer and more predictable.

Player wrapper updates for subtitle track selection:

• In LibMDK (lib_mdk.dart), modified setSubtitleTrack to check for both null and the default "no" subtitle stream, disabling external subtitles and returning -1 when appropriate.
• Cleaned up unreachable code in LibMDK's setSubtitleTrack by removing a redundant return statement.
• In LibMPV (lib_mpv.dart), consolidated the check for invalid subtitle streams (null or "no" stream), ensuring that the player disables subtitles and returns -1 in these cases, and moved the codec assignment after the check for clarity and correctness.
• Minor code cleanup in LibMPV to remove unnecessary braces after handling subtitle tracks.

Issue Being Fixed

Resolves #877

Screenshots / Recordings

Checklist

• If a new package was added, did you ensure it works for all supported platforms? Is the package well maintained
• Check that any changes are related to the issue at hand.

[PartyDonut]

Looks good to me, thanks