LUCENE-5961: Fix the exists() method for FunctionValues returned by many ValueSoures to behave properly when wrapping other ValueSources which do not exist for the specified document (merge r1632414)

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_5x@1632432 13f79535-47bb-0310-9956-ffa450edef68