OAK-1431: Handling of empty or invalid names

Make NodeState.getChildNode() and Tree.getChild() (but not hasChild) throw
IllegalArgumentException on invalid names. Adjust other pieces of code
accordingly.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1569561 13f79535-47bb-0310-9956-ffa450edef68