### 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)

rect.position.y = xsi[p_screen].y_org;
			rect.size.width = xsi[p_screen].width;
			rect.size.height = xsi[p_screen].height;
		} else {
			ERR_PRINT("Invalid screen index: " + itos(p_screen) + "(count: " + itos(count) + ").");
		}
		if (xsi) {
			XFree(xsi);
		}
	} else {
		int count = XScreenCount(x11_display);
		if (p_screen < count) {
			Window root = XRootWindow(x11_display, p_screen);
			XWindowAttributes xwa;
			XGetWindowAttributes(x11_display, root, &xwa);
			rect.position.x = xwa.x;
			rect.position.y = xwa.y;
			rect.size.width = xwa.width;
			rect.size.height = xwa.height;
		} else {
			ERR_PRINT("Invalid screen index: " + itos(p_screen) + "(count: " + itos(count) + ").");
		}
	}
	return rect;
}
Point2i DisplayServerX11::screen_get_position(int p_screen) const {
	_THREAD_SAFE_METHOD_
	return _screen_get_rect(p_screen).position;
}
Size2i DisplayServerX11::screen_get_size(int p_screen) const {