Optional moreLikeThis parameters

I have a project where Solr RequestHandlers (https://wiki.apache.org/solr/SolrRequestHandler) are used in order to set some default parameters. In one of these RequestHandlers we use the moreLikeThis component, unfortunately, some parameters are set to blank by the MoreLikeThis component.

This PR would allow not to send the "fl" and "qf" parameters in the query instead of forcing them to a blank string.