### 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)

m.message_type = xdnd_finished;
						m.format = 32;
						m.data.l[0] = windows[window_id].x11_window;
						m.data.l[1] = 0;
						m.data.l[2] = None; 
						XSendEvent(x11_display, event.xclient.data.l[0], False, NoEventMask, (XEvent *)&m);
					}
				}
				break;
			default:
				break;
		}
	}
	XFlush(x11_display);
	if (do_mouse_warp) {
		XWarpPointer(x11_display, None, windows[MAIN_WINDOW_ID].x11_window,
				0, 0, 0, 0, (int)windows[MAIN_WINDOW_ID].size.width / 2, (int)windows[MAIN_WINDOW_ID].size.height / 2);
		/*
		Window root, child;
		int root_x, root_y;
		int win_x, win_y;
		unsigned int mask;
		XQueryPointer( x11_display, x11_window, &root, &child, &root_x, &root_y, &win_x, &win_y, &mask );
		printf("Root: %d,%d\n", root_x, root_y);
		printf("Win: %d,%d\n", win_x, win_y);
		*/
	}
	Input::get_singleton()->flush_buffered_events();
}
void DisplayServerX11::release_rendering_thread() {