### 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_
	List<WindowID>::Element *E = popup_list.find(p_window);
	while (E) {
		List<WindowID>::Element *F = E->next();
		WindowID win_id = E->get();
		popup_list.erase(E);
		_send_window_event(windows[win_id], DisplayServerX11::WINDOW_EVENT_CLOSE_REQUEST);
		E = F;
	}
}
bool DisplayServerX11::mouse_process_popups() {
	_THREAD_SAFE_METHOD_
	if (popup_list.is_empty()) {
		return false;
	}
	uint64_t delta = OS::get_singleton()->get_ticks_msec() - time_since_popup;
	if (delta < 250) {
		return false;
	}
	int number_of_screens = XScreenCount(x11_display);
	bool closed = false;
	for (int i = 0; i < number_of_screens; i++) {
		Window root, child;
		int root_x, root_y, win_x, win_y;
		unsigned int mask;
		if (XQueryPointer(x11_display, XRootWindow(x11_display, i), &root, &child, &root_x, &root_y, &win_x, &win_y, &mask)) {
			XWindowAttributes root_attrs;
			XGetWindowAttributes(x11_display, root, &root_attrs);
			Vector2i pos = Vector2i(root_attrs.x + root_x, root_attrs.y + root_y);
			if (mask != last_mouse_monitor_mask) {