SOLR-6604: SOLR-6812: Fix NPE with distrib.singlePass=true and expand component. Increased test coverage of expand component with docValues.

This closes #98.

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1645098 13f79535-47bb-0310-9956-ffa450edef68