OAK-672: Avoid JCR APIs calling other JCR APIs

Move the "if (value != null)" logic from the internalSetProperty()
methods to higher up as it doesn't rely on any internal state
information (that could change) and reduces the complexity of those
methods. Most of the calling setProperty() methods in any case need
null checks to determine how to pre-process the value arguments.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1460979 13f79535-47bb-0310-9956-ffa450edef68