Handle case of no ingame music being available when menu music is

Because the menu music code relies on those tracks existing, this
is probably the best solution we can come up with.
Handle case of neither ingame nor menu music being available

Previously would throw IndexErrors when looking for `track` in an empty
sequence, now just remains silent even though the settings claim that
sound is enabled (which, as long as the sound effects exist, is true
technically).
Comment and whitespace fixes
Reorder imports
Merge pull request #2038 from unknown-horizons/missing-music

Fix crashes caused by missing music files.

If for some reason we don't ship all or any of our music tracks, UH shouldn't crash still.

Maybe useful for packages with reduced size, but also allows players to remove tracks they don't like to get their favorite track(s) played more often.