Fix type of notification_func

[Sauermann]

fix comment #78634 (comment)
cc @dsnopek

[Gallilus]

Pulled the Gun too early with my approval. My issue seems solved by initializing to nullptr.

struct {
		GDExtensionScriptInstanceNotification notification_func = nullptr;
	} deprecated_native_info;

[dsnopek]

@Gallilus Ack, I think you're right! We're only initializing that value if you call script_instance_create, but not if you call script_instance_create2. Good catch!

@Sauermann If you don't mind adding the = nullptr, we can fix two problems at once :-)

[Sauermann]

Of course. done.

[dsnopek]

Thanks!

[akien-mga]

Thanks!