ResultGenerator, SolrDocPrimarySearch: Always emit at least one CAS

Sometimes, a Solr search wouldn't yield any results, causing no CASes
being emitted. However, the AnswerMerger must see three CASes with
isLast flags and will not emit answers otherwise; therefore, after
addition of multiple search sources, we are seeing plummetting
performance as in some cases we would yield no answers due to this.
doc/TODO: Slight reorganization, more TODO items