### 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)

if (windows[window_id].mpass) {
				return HTTRANSPARENT;
			}
		} break;
		case WM_MOUSEACTIVATE: {
			if (windows[window_id].no_focus) {
				return MA_NOACTIVATEANDEAT; 
			} else if (windows[window_id].is_popup) {
				return MA_NOACTIVATE; 
			}
		} break;
		case WM_SETFOCUS: {
			windows[window_id].window_has_focus = true;
			last_focused_window = window_id;
			
			_set_mouse_mode_impl(mouse_mode);
			if (!app_focused) {
				if (OS::get_singleton()->get_main_loop()) {
					OS::get_singleton()->get_main_loop()->notification(MainLoop::NOTIFICATION_APPLICATION_FOCUS_IN);
				}
				app_focused = true;
			}
		} break;
		case WM_KILLFOCUS: {
			windows[window_id].window_has_focus = false;
			last_focused_window = window_id;
			
			ReleaseCapture();
			
			for (const KeyValue<int, Vector2> &E : touch_state) {