### 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)

_THREAD_SAFE_METHOD_
	bool has_popup_ancestor = false;
	WindowID transient_root = p_window;
	while (true) {
		WindowID parent = windows[transient_root].transient_parent;
		if (parent == INVALID_WINDOW_ID) {
			break;
		} else {
			transient_root = parent;
			if (windows[parent].is_popup) {
				has_popup_ancestor = true;
				break;
			}
		}
	}
	WindowData &wd = windows[p_window];
	if (wd.is_popup || has_popup_ancestor) {
		
		List<WindowID>::Element *C = nullptr;
		List<WindowID>::Element *E = popup_list.back();
		while (E) {
			if (wd.transient_parent != E->get() || wd.transient_parent == INVALID_WINDOW_ID) {
				C = E;
				E = E->prev();
			} else {
				break;
			}
		}
		if (C) {