### 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)

app_focused = false;
			}
		} else {
			time_since_no_focus = OS::get_singleton()->get_ticks_msec();
		}
	}
	do_mouse_warp = false;
	
	bool mouse_mode_grab = mouse_mode == MOUSE_MODE_CAPTURED || mouse_mode == MOUSE_MODE_CONFINED || mouse_mode == MOUSE_MODE_CONFINED_HIDDEN;
	xi.pressure = 0;
	xi.tilt = Vector2();
	xi.pressure_supported = false;
	LocalVector<XEvent> events;
	{
		
		MutexLock mutex_lock(events_mutex);
		events = polled_events;
		polled_events.clear();
		
		_check_pending_events(events);
	}
	for (uint32_t event_index = 0; event_index < events.size(); ++event_index) {
		XEvent &event = events[event_index];
		bool ime_window_event = false;
		WindowID window_id = MAIN_WINDOW_ID;
		
		for (const KeyValue<WindowID, WindowData> &E : windows) {
			if (event.xany.window == E.value.x11_window) {
				window_id = E.key;
				break;