lastfm-albumart: Do not return default image
lastfm-albumart: Do not return default image

When Last.fm does not recognize an album, it returns a default image to be used
as the thumbnail. This prevents other sources to take the chance to try to
provide a real thumbnail.

Do not return a default thumbnail if the source can not solve it, and let other
sources to try it.

MikePetullo/grilo-plugins,kyoushuu/grilo-plugins,vrutkovs/grilo-plugins,vrutkovs/grilo-plugins,GNOME/grilo-plugins,MikePetullo/grilo-plugins,jasuarez/grilo-plugins,kyoushuu/grilo-plugins,jasuarez/grilo-plugins,kyoushuu/grilo-plugins,vrutkovs/grilo-plugins,MathieuDuponchelle/grilo-plugins,MikePetullo/grilo-plugins,MathieuDuponchelle/grilo-plugins,MathieuDuponchelle/grilo-plugins,grilofw/grilo-plugins,vrutkovs/grilo-plugins,GNOME/grilo-plugins,MathieuDuponchelle/grilo-plugins,grilofw/grilo-plugins