Fix databasejoin onSave error

This fix is provided by bauer and fixes the following error:

PHP Warning: Missing argument 1 for PlgFabrik_Element:: onSave(), called in /home/public_html/plugins/fabrik_element/databasejoin/databasejoin.php on line 2407 and defined in /home/public_html/components/com_fabrik/models/element.php on line 3830.