Binding Specification: Type empty needs better representation than Boolean or Null vs NonNull

In Binding specification v1 YANG type empty was represented as Boolean, which was bit confusing. In normalized Node it is represented as java.lang.Void type which is bit unusual type to work with in normal code path.

Type empty needs better representation which will clearly describe it semantics:
Leaf present. The best way is to introduce enumeration with value PRESENT
eg. Empty.PRESENT and null could still be not present.