### 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)

int event_base, error_base;
	if (XineramaQueryExtension(x11_display, &event_base, &error_base)) {
		return 0;
	} else {
		return XDefaultScreen(x11_display);
	}
}
int DisplayServerX11::get_keyboard_focus_screen() const {
	int count = get_screen_count();
	if (count < 2) {
		
		return 0;
	}
	Window focus = 0;
	int revert_to = 0;
	XGetInputFocus(x11_display, &focus, &revert_to);
	if (focus) {
		Window focus_child = 0;
		int x = 0, y = 0;
		XTranslateCoordinates(x11_display, focus, DefaultRootWindow(x11_display), 0, 0, &x, &y, &focus_child);
		XWindowAttributes xwa;