allow artistId to be blank [#150]
allow artistId to be blank [#150]

TheOpenDevProject/gmusicapi,dvirtz/gmusicapi,simon-weber/gmusicapi,dvirtz/gmusicapi,simon-weber/Unofficial-Google-Music-API,peetahzee/Unofficial-Google-Music-API,thebigmunch/gmusicapi,tanhaiwang/gmusicapi