Made $old_instance param optional due to warning

Warning: Missing argument 2 for WPSpin\PlaylistWidgetView::form(), called in /path/to/wordpress/wp-includes/widgets.php on line 270 and defined in /path/to/wordpress/wp-content/plugins/WPSpin/viewmodels/playlistwidgetview.php on line 31

Word of warning: I don't fully understand how this second parameter is being used so I don't know what other ramifications this change will have.