tracker-extract: Only extract media art if artist or title is known
tracker-extract: Only extract media art if artist or title is known

media_art_process_*() require at least one of the track’s artist and
title to be specified, and will fail an assertion precondition if both
are NULL.

Other tracker-extract-*.c media backends already do this check, but it
was missing from the GStreamer extractor.

https://bugzilla.gnome.org/show_bug.cgi?id=748227

outofbits/tracker,outofbits/tracker,outofbits/tracker,outofbits/tracker,outofbits/tracker,outofbits/tracker,outofbits/tracker