Include nullability information

[raniejade]

It will be nice to have a field that denotes if the returned type of a property or method can be null. This is useful for language bindings that support nullable types (i.e Kotlin).

[aaronfranke]

Ideally what we want is to let the core API provide this information. Therefore, this depends on godotengine/godot-proposals#162, since it will be easier to implement this for GDNative after it's supported in GDScript.

[BastiaanOlij]

@raniejade I'm for closing this issue. This is not a godot-headers issue but something that needs to be supported in Godot itself first (as per proposal @aaronfranke is pointing at).

In the meanwhile I would make the assumption that all methods that return objects can return either a null pointer or null reference. It's pretty standard in the Godot core to return this in failure situations.