### 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), 0);
	switch (p_handle_type) {
		case DISPLAY_HANDLE: {
			return (int64_t)x11_display;
		}
		case WINDOW_HANDLE: {
			return (int64_t)windows[p_window].x11_window;
		}
		case WINDOW_VIEW: {
			return 0; 
		}
#ifdef GLES3_ENABLED
		case OPENGL_CONTEXT: {
			if (gl_manager) {
				return (int64_t)gl_manager->get_glx_context(p_window);
			}
			return 0;
		}
#endif
		default: {
			return 0;
		}
	}
}
void DisplayServerX11::window_attach_instance_id(ObjectID p_instance, WindowID p_window) {
	ERR_FAIL_COND(!windows.has(p_window));
	WindowData &wd = windows[p_window];
	wd.instance_id = p_instance;
}
ObjectID DisplayServerX11::window_get_attached_instance_id(WindowID p_window) const {