### 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)

if (((mask & Button1Mask) || (mask & Button2Mask) || (mask & Button3Mask) || (mask & Button4Mask) || (mask & Button5Mask))) {
					List<WindowID>::Element *C = nullptr;
					List<WindowID>::Element *E = popup_list.back();
					
					while (E) {
						
						Rect2i win_rect = Rect2i(window_get_position(E->get()), window_get_size(E->get()));
						
						Rect2i safe_rect = window_get_popup_safe_rect(E->get());
						if (win_rect.has_point(pos)) {
							break;
						} else if (safe_rect != Rect2i() && safe_rect.has_point(pos)) {
							break;
						} else {
							C = E;
							E = E->prev();
						}
					}
					if (C) {
						_send_window_event(windows[C->get()], DisplayServerX11::WINDOW_EVENT_CLOSE_REQUEST);
						closed = true;
					}
				}
			}
			last_mouse_monitor_mask = mask;
		}
	}
	return closed;
}
void DisplayServerX11::process_events() {