Fix for #1109

When Vinyl adds a song to the MemCache, and the artist name list is empty, the album ID was being inserted into albumsByAlbumIdAndArtistId, but no artists inside it. Then, empty albums were captured by Discography.getAllAlbums, and they all used the same ID (Album.safeGetFirstSong().albumId == -1).

The lines below, paired with the absence of any Artists on the song is what ultimately caused the "Unknown Album" issue.

if (albumsByArtist == null) {
    albumsByArtist = new HashMap<>();
    albumsByAlbumIdAndArtistId.put(song.albumId, albumsByArtist);
}

The unit test implemented corroborates this PR, and it would not pass with the current version of Vinyl.

So the error was not that the albums were invalid (and thus should have been delivered), but that these were real albums that should have been displayed.