Fixed a minor bug encountered in getNodes call with -1 (limitless) depth

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1443497 13f79535-47bb-0310-9956-ffa450edef68
OAK-613 NPE in Property2IndexDiff#getIndexes(String) when processing a removed node.
 - Test for the NPE guard

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1443498 13f79535-47bb-0310-9956-ffa450edef68