YouTube Music: keep album tracks that have no artist id (#6115)

_This issue was auto triaged._

# What does this implement/fix?

Some YouTube Music album tracks come back with an artist that has no id
at all (`{'id': None, 'name': 'Lost Frequencies & Zak Abel'}`), which
happens mostly on collaborations. `_parse_track` skips artists without
an id or channelId, ends up with an empty artist list and raises
`InvalidDataError`, and `get_album_tracks` swallows that and moves on.
The result is that those tracks silently vanish from the album listing —
the reported album is missing tracks 1, 4, 7 and 11.

YouTube Music's own UI falls back to the album artist for these tracks,
so we now do the same: when none of a track's artists can be mapped,
credit the track to the album artists instead of dropping it.

**Related issue (if applicable):**

- related issue https://github.com/music-assistant/support/issues/6266

## Types of changes

- [x] Bugfix (non-breaking change which fixes an issue) — `bugfix`
- [ ] New feature (non-breaking change which adds functionality) —
`new-feature`
- [ ] Enhancement to an existing feature — `enhancement`
- [ ] New music/player/metadata/plugin provider — `new-provider`
- [ ] Breaking change (fix or feature that would cause existing
functionality to not work as expected) — `breaking-change`
- [ ] Refactor (no behaviour change) — `refactor`
- [ ] Documentation only — `documentation`
- [ ] Maintenance / chore — `maintenance`
- [ ] CI / workflow change — `ci`
- [ ] Dependencies bump — `dependencies`

## Checklist

- [x] The code change is tested and works locally.
- [x] `pre-commit run --all-files` passes.
- [x] `pytest` passes, and tests have been added/updated under `tests/`
where applicable.
- [ ] For changes to shared models, the companion PR in
`music-assistant/models` is linked.
- [ ] For changes affecting the UI, the companion PR in
`music-assistant/frontend` is linked.
- [x] I have read and complied with the project's [AI
Policy](https://github.com/music-assistant/.github/blob/main/AI_POLICY.md)
for any AI-assisted contributions.
- [ ] I have [raised a PR against the documentation
repository](https://github.com/music-assistant/music-assistant.io/blob/main/CONTRIBUTING.md)
targeting the main or beta branch as appropriate.