SolrFeature: no-fq optimisation

Stanislav Livotov wrote in SOLR-12688:

... SolrFeature was not optimally implemented for the case when no fq parameter was passed. I'm not absolutely sure what was the intention to introduce both q(which is supposed to be a function query) and fq parameter for the same SolrFeature at all(Is there a case when they will be used together ? ), so I decided not to change behavior but just optimize described case ...

(Please see SOLR-12688 description for overall context and analysis results.)