Restriction of the nodes api to clearly define never-null function arguments.
Side effects: some assertions and null-check (with early return) were removed.
On the caller side is ensured to never derefer null to pass argument (mainly in RNA).
In addition, one pointer argument now actually a return type.
By-reference return types instead of pointers going to be separate kind of
change since also imply of cleaning up variables created from reference.
Also good future improvement would be to mark a copy-constructor as
explicit for DNA node types.
Pull Request: https://projects.blender.org/blender/blender/pulls/134627
119fc054f8