### 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)

mb.instantiate();
				mb->set_window_id(window_id);
				_get_key_modifier_state(event.xbutton.state, mb);
				mb->set_button_index((MouseButton)event.xbutton.button);
				if (mb->get_button_index() == MouseButton::RIGHT) {
					mb->set_button_index(MouseButton::MIDDLE);
				} else if (mb->get_button_index() == MouseButton::MIDDLE) {
					mb->set_button_index(MouseButton::RIGHT);
				}
				mb->set_button_mask(_get_mouse_button_state(mb->get_button_index(), event.xbutton.type));
				mb->set_position(Vector2(event.xbutton.x, event.xbutton.y));
				mb->set_global_position(mb->get_position());
				mb->set_pressed((event.type == ButtonPress));
				const WindowData &wd = windows[window_id];
				if (event.type == ButtonPress) {
					DEBUG_LOG_X11("[%u] ButtonPress window=%lu (%u), button_index=%u \n", frame, event.xbutton.window, window_id, mb->get_button_index());
					
					
					
					if (!wd.no_focus && !wd.is_popup) {
						XSetInputFocus(x11_display, wd.x11_window, RevertToPointerRoot, CurrentTime);
					}
					uint64_t diff = OS::get_singleton()->get_ticks_usec() / 1000 - last_click_ms;
					if (mb->get_button_index() == last_click_button_index) {
						if (diff < 400 && Vector2(last_click_pos).distance_to(Vector2(event.xbutton.x, event.xbutton.y)) < 5) {
							last_click_ms = 0;
							last_click_pos = Point2i(-100, -100);
							last_click_button_index = MouseButton::NONE;
							mb->set_double_click(true);
						}