tracker-extract: Handle artist=null use-case for cover.jpg/png too
tracker-extract: Handle artist=null use-case for cover.jpg/png too

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