### 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)

} else if (mb->get_button_index() < MouseButton::WHEEL_UP || mb->get_button_index() > MouseButton::WHEEL_RIGHT) {
						last_click_button_index = mb->get_button_index();
					}
					if (!mb->is_double_click()) {
						last_click_ms += diff;
						last_click_pos = Point2i(event.xbutton.x, event.xbutton.y);
					}
				} else {
					DEBUG_LOG_X11("[%u] ButtonRelease window=%lu (%u), button_index=%u \n", frame, event.xbutton.window, window_id, mb->get_button_index());
					WindowID window_id_other = INVALID_WINDOW_ID;
					Window wd_other_x11_window;
					if (wd.focused) {
						
						WindowID popup_id = _get_focused_window_or_popup();
						if (popup_id != INVALID_WINDOW_ID && popup_id != window_id) {
							window_id_other = popup_id;
							wd_other_x11_window = windows[popup_id].x11_window;
						}
					} else {
						
						
						
						
						
						for (const KeyValue<WindowID, WindowData> &E : windows) {
							if (E.value.focused) {
								if (E.key != window_id) {
									window_id_other = E.key;
									wd_other_x11_window = E.value.x11_window;
								}