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
Merge pull request #759 from Sophist-UK/bauer_fix_databasejoin_onSave

Fix databasejoin onSave error