Provide failsafe against unavailable props

[robbevp]

If a related property is not available, the app tends to freeze or components will not show up.
This already happens when an object is destroyed but is still refered to on another object (see #77 ), or might happen when loading objects is stuck (for example, due to a network error, see #65).

Once we have #75 merged, this will only become more common, since we won't know the order in which objects are added to the store.

Proposed changes

• provide a fallback in components (TrackGenres, TrackArtists and AlbumArtists) so that they get displayed even if some of the related objects are missing.
• provide a fallback in getters (most notably Tracks/tracksByAlbumAndNumber where most of the critical errors seem to occur) so that it will always output something

[chvp]

This happened when testing #44 and disabled updates for the flagged track count. An album wasn't found. Trace:

TypeError: "rootState.albums.albums[a1.album_id] is undefined"
    tracksFlagged tracks.js:168
    tracksFlagged tracks.js:166
    wrappedGetter vuex.esm.js:822
    partial vuex.esm.js:99
    VueJS 3
    get vuex.esm.js:612
    numberOfFlaggedItems store.js:67
    wrappedGetter vuex.esm.js:822
    partial vuex.esm.js:99
    VueJS 3
    get vuex.esm.js:612
    mappedGetter vuex.esm.js:967
    VueJS 4
    render App.vue:231
    VueJS 7