### 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)

ERR_FAIL_COND_V(!windows.has(p_window), ObjectID());
	const WindowData &wd = windows[p_window];
	return wd.instance_id;
}
DisplayServerX11::WindowID DisplayServerX11::get_window_at_screen_position(const Point2i &p_position) const {
	WindowID found_window = INVALID_WINDOW_ID;
	WindowID parent_window = INVALID_WINDOW_ID;
	unsigned int focus_order = 0;
	for (const KeyValue<WindowID, WindowData> &E : windows) {
		const WindowData &wd = E.value;
		
		if (wd.focus_order == 0) {
			continue;
		}
		
		WindowID window_id = E.key;
		Rect2i win_rect = Rect2i(window_get_position(window_id), window_get_size(window_id));
		if (win_rect.has_point(p_position)) {
			
			if ((parent_window != wd.transient_parent) || (wd.focus_order > focus_order)) {
				found_window = window_id;
				parent_window = wd.transient_parent;
				focus_order = wd.focus_order;
			}
		}
	}
	return found_window;
}
void DisplayServerX11::window_set_title(const String &p_title, WindowID p_window) {