### 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_
#ifdef DISPLAY_SERVER_X11_DEBUG_LOGS_ENABLED
	static int frame = 0;
	++frame;
#endif
	bool ignore_events = mouse_process_popups();
	if (app_focused) {
		
		bool focus_found = false;
		for (const KeyValue<WindowID, WindowData> &E : windows) {
			if (E.value.focused) {
				focus_found = true;
				break;
			}
		}
		if (!focus_found) {
			uint64_t delta = OS::get_singleton()->get_ticks_msec() - time_since_no_focus;
			if (delta > 250) {
				
				if (OS::get_singleton()->get_main_loop()) {
					DEBUG_LOG_X11("All focus lost, triggering NOTIFICATION_APPLICATION_FOCUS_OUT\n");
					OS::get_singleton()->get_main_loop()->notification(MainLoop::NOTIFICATION_APPLICATION_FOCUS_OUT);
				}