### 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)

} else {
						count++;
					}
				}
			}
			if (count > 0) {
				ds->im_selection = Point2i(start + call_data->chg_first, count);
			} else {
				ds->im_selection = Point2i(call_data->caret, 0);
			}
		} else {
			ds->im_text = String();
			ds->im_selection = Point2i();
		}
		OS_Unix::get_singleton()->get_main_loop()->notification(MainLoop::NOTIFICATION_OS_IME_UPDATE);
	}
}
void DisplayServerX11::_xim_preedit_caret_callback(::XIM xim, ::XPointer client_data,
		::XIMPreeditCaretCallbackStruct *call_data) {
}
void DisplayServerX11::_xim_destroy_callback(::XIM im, ::XPointer client_data,
		::XPointer call_data) {
	WARN_PRINT("Input method stopped");
	DisplayServerX11 *ds = reinterpret_cast<DisplayServerX11 *>(client_data);
	ds->xim = nullptr;
	for (KeyValue<WindowID, WindowData> &E : ds->windows) {
		E.value.xic = nullptr;
	}
}
void DisplayServerX11::_window_changed(XEvent *event) {