### 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)

String ret;
	Atom utf8_atom = XInternAtom(x11_display, "UTF8_STRING", True);
	if (utf8_atom != None) {
		ret = _clipboard_get_impl(p_source, x11_window, utf8_atom);
	}
	if (ret.is_empty()) {
		ret = _clipboard_get_impl(p_source, x11_window, XA_STRING);
	}
	return ret;
}
String DisplayServerX11::clipboard_get() const {
	_THREAD_SAFE_METHOD_
	String ret;
	ret = _clipboard_get(XInternAtom(x11_display, "CLIPBOARD", 0), windows[MAIN_WINDOW_ID].x11_window);
	if (ret.is_empty()) {
		ret = _clipboard_get(XA_PRIMARY, windows[MAIN_WINDOW_ID].x11_window);
	}
	return ret;
}
String DisplayServerX11::clipboard_get_primary() const {
	_THREAD_SAFE_METHOD_
	String ret;
	ret = _clipboard_get(XInternAtom(x11_display, "PRIMARY", 0), windows[MAIN_WINDOW_ID].x11_window);
	if (ret.is_empty()) {
		ret = _clipboard_get(XA_PRIMARY, windows[MAIN_WINDOW_ID].x11_window);
	}
	return ret;
}
Bool DisplayServerX11::_predicate_clipboard_save_targets(Display *display, XEvent *event, XPointer arg) {