fix restore of tracks with missing metadata in playlist

Summary:
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

Test Plan: new test is OK and a track without album, track and disc numbers will restore and playback will restore at correct position

Reviewers: Elisa, astippich

Reviewed By: astippich

Subscribers: astippich, Elisa

Tags: Elisa

Differential Revision: https://phabricator.kde.org/D21900