### 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_
	ERR_FAIL_COND(!windows.has(p_window));
	WindowData &wd = windows[p_window];
	if (!wd.xic) {
		return;
	}
	if (!wd.focused) {
		wd.ime_active = false;
		im_text = String();
		im_selection = Vector2i();
		return;
	}
	
	
	if (p_active) {
		MutexLock mutex_lock(events_mutex);
		wd.ime_active = true;
		XMapWindow(x11_display, wd.x11_xim_window);
		XWindowAttributes xwa;
		XSync(x11_display, False);
		XGetWindowAttributes(x11_display, wd.x11_xim_window, &xwa);
		if (xwa.map_state == IsViewable) {
			XSetInputFocus(x11_display, wd.x11_xim_window, RevertToParent, CurrentTime);
		}