query: Don't assume that nmm:MusicPieces will have a nie:url in playlists

nmm:MusicPieces may have an indirect relation to the nfo:FileDataObject
and its nie:url, expressed through the nie:isStoredAs relationship. As
such, poking the nie:url of nmm:MusicPieces themselves may bring these
queries to bring 0 results unexpectedly.

https://bugzilla.gnome.org/show_bug.cgi?id=763304