OAK-781: Clarify / fix effects of MISSING_NODE as base state of NodeBuilder

Replace null values in the nodes maps of Mutable- and ModifiedNodeState with non-existing NodeState instances

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1477716 13f79535-47bb-0310-9956-ffa450edef68