### Godot version

Godot Engine v4.0.stable.official.92bee43ad

### System information

Windows 10

### Issue description

The documentation states that passing ```null``` into the image parameter of ```set_custom_mouse_cursor``` will change it back to the system cursor, however I find that it removes the cursor's image entirely, rather than setting it to the system's cursor.


### Steps to reproduce

```var isimage = false
var theimage = load("res://final.png")

func _on_my_button_pressed():
    if isimage == false:
        Input.set_custom_mouse_cursor(theimage, Input.CURSOR_ARROW, Vector2(16,16))
        isimage = true
        
    else:
        Input.set_custom_mouse_cursor(null)
        isimage = false```

When the button is pressed, the cursor is meant to change to the new cursor. When it has been pressed a second time, it changes back to the default cursor. However, upon pressing it a second time, it simply disappears. The cursor becomes visible again when the cursor exits and re-enters the window.

### Minimal reproduction project

[null mouse.zip](https://github.com/godotengine/godot/files/10901969/null.mouse.zip)

const List<WindowID>::Element *E = popup_list.back();
	if (E) {
		return E->get();
	}
	return last_focused_window;
}
void DisplayServerX11::_dispatch_input_events(const Ref<InputEvent> &p_event) {
	static_cast<DisplayServerX11 *>(get_singleton())->_dispatch_input_event(p_event);
}
void DisplayServerX11::_dispatch_input_event(const Ref<InputEvent> &p_event) {
	Variant ev = p_event;
	Variant *evp = &ev;
	Variant ret;
	Callable::CallError ce;
	{
		List<WindowID>::Element *E = popup_list.back();
		if (E && Object::cast_to<InputEventKey>(*p_event)) {
			
			if (windows.has(E->get())) {
				Callable callable = windows[E->get()].input_event_callback;
				if (callable.is_valid()) {
					callable.callp((const Variant **)&evp, 1, ret, ce);
				}
			}
			return;
		}