OAK-598: TreeLocation#NULL allows to reveal the existence of items that are not accessible

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1440930 13f79535-47bb-0310-9956-ffa450edef68
Javadoc: use @code instead of </code>

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1440931 13f79535-47bb-0310-9956-ffa450edef68