No notification if the song has no album art

[ColinDuquesnoy]

There should be a notification even if there is no album art. In that case, using the application logo might be good.

The current implementation waits for the album art to be ready (sometimes, depending on the service, the url is initially empty and set a few milliseconds later). If the albumArt url is never set, then it will wait forever. The solution is to add a small timeout to display the notification even if the artUrl is empty.