The getSong API endpoint is used by, e.g., the subsonic plugin to mopidy.

Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>

If the name of an artist is not set, the serialiser will crash. Instead,
just skip such an artist when serialising a list of artists.

Also add test for serialising an artist with an empty name.

Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>