lastgenre: force real lookup during import only if no custom whitelist set

I activate lastgenre plugin on import so it fetches correct genre if I haven't done it already.
Forcing real lookup in all cases had bad side-effect in my workflow as it overruled a (allowed) genre that I had picked manually by another (allowed) less relevant one.

The reasoning of this PR is to consider that if user defines a custom whitelist and current track is allowed then it's highly likely that it had been manually set and a change is not desired.