select?collapse=... - fix NPE in Collapsing(FieldValue|Score)Collector

https://issues.apache.org/jira/i#browse/SOLR-6510

Patch is against branch_4x where CollapsingQParserPlugin line 230's searcher.getAtomicReader().getSortedDocValues call could return null. On trunk the line 230 call has been replaced (by LUCENE-5666) and so the CollapsingQParserPlugin part of the patch is not required.

The TestCollapseQParserPlugin change could go to branch_4x only or branch_4x and trunk.