fix storing of tracks without track, disc, ...
correctly store a null when track, disc, channels, bit rate, sample rate
are not existing in the track data
will need a complete reindexing to fix users database
let play list and audio player restore state when track has few metadata
when album, track number or disc number are missing, the state is not
correctly restored at startup
let those metadata be optional in the saved state such that tracks
missing those are still restored