Use album artist for empty artist tag

[smithjd15]

Use the album artist instead if the artist tag is empty.

[MaxKellermann]

Doesn't compile:

../../src/Main.cxx: In member function ‘void Instance::OnMpdSongChanged(const mpd_song*)’:
../../src/Main.cxx:79:28: error: invalid conversion from ‘const char*’ to ‘char*’ [-fpermissive]
   79 |   artist = mpd_song_get_tag(song, MPD_TAG_ARTIST, 0);

Also please don't add redundant code. Make a function which returns the artist with fallbacks, and call that, instead of copying code around.

[smithjd15]

Sorry, all fixed.