### 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)

Point2i offset = _get_screens_origin();
	POINT p;
	p.x = p_position.x + offset.x;
	p.y = p_position.y + offset.y;
	HWND hwnd = WindowFromPoint(p);
	for (const KeyValue<WindowID, WindowData> &E : windows) {
		if (E.value.hWnd == hwnd) {
			return E.key;
		}
	}
	return INVALID_WINDOW_ID;
}
DisplayServer::WindowID DisplayServerWindows::create_sub_window(WindowMode p_mode, VSyncMode p_vsync_mode, uint32_t p_flags, const Rect2i &p_rect) {
	_THREAD_SAFE_METHOD_
	WindowID window_id = _create_window(p_mode, p_vsync_mode, p_flags, p_rect);
	ERR_FAIL_COND_V_MSG(window_id == INVALID_WINDOW_ID, INVALID_WINDOW_ID, "Failed to create sub window.");
	WindowData &wd = windows[window_id];
	if (p_flags & WINDOW_FLAG_RESIZE_DISABLED_BIT) {
		wd.resizable = false;
	}
	if (p_flags & WINDOW_FLAG_BORDERLESS_BIT) {
		wd.borderless = true;
	}
	if (p_flags & WINDOW_FLAG_ALWAYS_ON_TOP_BIT && p_mode != WINDOW_MODE_FULLSCREEN && p_mode != WINDOW_MODE_EXCLUSIVE_FULLSCREEN) {
		wd.always_on_top = true;
	}
	if (p_flags & WINDOW_FLAG_NO_FOCUS_BIT) {