### 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)

_THREAD_SAFE_METHOD_
	WindowID id = _create_window(p_mode, p_vsync_mode, p_flags, p_rect);
	for (int i = 0; i < WINDOW_FLAG_MAX; i++) {
		if (p_flags & (1 << i)) {
			window_set_flag(WindowFlags(i), true, id);
		}
	}
	return id;
}
void DisplayServerX11::show_window(WindowID p_id) {
	_THREAD_SAFE_METHOD_
	const WindowData &wd = windows[p_id];
	popup_open(p_id);
	DEBUG_LOG_X11("show_window: %lu (%u) \n", wd.x11_window, p_id);
	XMapWindow(x11_display, wd.x11_window);
	XSync(x11_display, False);
	_validate_mode_on_map(p_id);
}
void DisplayServerX11::delete_sub_window(WindowID p_id) {
	_THREAD_SAFE_METHOD_
	ERR_FAIL_COND(!windows.has(p_id));
	ERR_FAIL_COND_MSG(p_id == MAIN_WINDOW_ID, "Main window can't be deleted");
	popup_close(p_id);
	WindowData &wd = windows[p_id];
	DEBUG_LOG_X11("delete_sub_window: %lu (%u) \n", wd.x11_window, p_id);
	while (wd.transient_children.size()) {
		window_set_transient(*wd.transient_children.begin(), INVALID_WINDOW_ID);
	}
	if (wd.transient_parent != INVALID_WINDOW_ID) {