OAK-37: Use nullability annotation to enforce/document API contract

Upgrade to newer Findbugs version to avoid http://sourceforge.net/p/findbugs/bugs/1043/

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1457701 13f79535-47bb-0310-9956-ffa450edef68