SOLR-6626 NPE in FieldMutatingUpdateProcessor when indexing a doc with 

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1644865 13f79535-47bb-0310-9956-ffa450edef68
Fixed typo in comment

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1644911 13f79535-47bb-0310-9956-ffa450edef68