Merge: modelbuilder_base: Define ANode as nullable

Merge: modelbuilder_base: Define ANode as nullable

Definition of ANode as nullable. When you create entities manually there is not necessarily a current attachment point (in the ast). It is therefore more practical to provide a service with the ANode which can be null.

In addition, the `fun error (n: nullable ANode, text: String)` method that will be called already handles this case.

Pull-Request: #2816