Object::cast_to checks provided object for nullptr

[DNKpp]

As Objec::cast_to is merely a dynamic_cast, it shouldn't fail on nullptr. As the provided argument gets dereferenced right away, we have to make the initial check. As I'm fairly new to the godot scene, I do not have much insights into the code base. Perhaps there is a more elegant solution to the problem, but I won't find it right now.

Nevertheless, this fix also fixes calls to Node::get_node<T> when no Node with the given name can be found (and probably even more other parts).

[Faless]

Looks good to me, and in line with what we do in godot. 👍

[Faless]

Thanks!