SOLR-4877, LUCENE-5023: Removed SolrIndexSearcher#getDocSetNC()'s special case for handling TermQuery to prevent NullPointerException if reader does not have fields

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1487914 13f79535-47bb-0310-9956-ffa450edef68