SOLR-6626 NPE in FieldMutatingUpdateProcessor when indexing a doc with null field value

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_5x@1644866 13f79535-47bb-0310-9956-ffa450edef68
Fixed typo in comment

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_5x@1644912 13f79535-47bb-0310-9956-ffa450edef68