Fix return type of Control::get_drag_data in API json

Control::get_drag_data is a virtual method that returns Variant, but have the
Object type in its MethodInfo.

This changes the MethodInfo of Control::get_drag_data

scene/gui/control.cpp

@@ -2918,7 +2918,11 @@ void Control::_bind_methods() {
 
 	BIND_VMETHOD(MethodInfo("_gui_input", PropertyInfo(Variant::OBJECT, "event", PROPERTY_HINT_RESOURCE_TYPE, "InputEvent")));
 	BIND_VMETHOD(MethodInfo(Variant::VECTOR2, "_get_minimum_size"));
-	BIND_VMETHOD(MethodInfo(Variant::OBJECT, "get_drag_data", PropertyInfo(Variant::VECTOR2, "position")));
+
+	MethodInfo get_drag_data = MethodInfo("get_drag_data", PropertyInfo(Variant::VECTOR2, "position"));
+	get_drag_data.return_val.usage |= PROPERTY_USAGE_NIL_IS_VARIANT;
+	BIND_VMETHOD(get_drag_data);
+
 	BIND_VMETHOD(MethodInfo(Variant::BOOL, "can_drop_data", PropertyInfo(Variant::VECTOR2, "position"), PropertyInfo(Variant::NIL, "data")));
 	BIND_VMETHOD(MethodInfo("drop_data", PropertyInfo(Variant::VECTOR2, "position"), PropertyInfo(Variant::NIL, "data")));
 	BIND_VMETHOD(MethodInfo(Variant::OBJECT, "_make_custom_tooltip", PropertyInfo(Variant::STRING, "for_text")));