OPENJPA-1794: Return null, rather than 0, on MAX function - back ported to 2.2.1.x Jeremy Bauer's commit to trunk.

git-svn-id: https://svn.apache.org/repos/asf/openjpa/branches/2.2.1.x@1529267 13f79535-47bb-0310-9956-ffa450edef68