DerivableSchemaNode API incorrect Javadoc

Interface DerivableSchemaNode, method getOriginal() contains incorrect Javadoc:

@return original node definition from grouping if this node is added by uses, null otherwise

Any implementation of this method never returns null. In fact, it returns Original.absent in case when parameter is null.

I propose this change:

@return original node definition from grouping if this node is added by uses, Optional.absent otherwise