PHP8: Add required 3rd param for Joomla

[skurvish]

With PHP8 when trying to unpublish a group, get the following error:

Fatal error: Declaration of & FabrikAdminControllerGroups::getModel($name = 'Group', $prefix = 'FabrikAdmi...') must be compatible with Joomla\CMS\MVC\Controller\BaseController::getModel($name = '', $prefix = '', $config = []) in /var/www/html/administrator/components/com_fabrik/controllers/groups.php on line 48

This PR adds the config parameter

[trob]

All other controllers are doing
public function getModel($name = 'List', $prefix = 'FabrikAdminModel', $config = array())
so maybe you can change this?

[skurvish]

Done.