### 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)

if (screen_is_kept_on() == p_enable) {
		return;
	}
	if (p_enable) {
		screensaver->inhibit();
	} else {
		screensaver->uninhibit();
	}
	keep_screen_on = p_enable;
}
bool DisplayServerX11::screen_is_kept_on() const {
	return keep_screen_on;
}
#endif
Vector<DisplayServer::WindowID> DisplayServerX11::get_window_list() const {
	_THREAD_SAFE_METHOD_
	Vector<int> ret;
	for (const KeyValue<WindowID, WindowData> &E : windows) {
		ret.push_back(E.key);
	}
	return ret;
}
DisplayServer::WindowID DisplayServerX11::create_sub_window(WindowMode p_mode, VSyncMode p_vsync_mode, uint32_t p_flags, const Rect2i &p_rect) {