gets first subcategory of a main category and redirect to this.uses an array, which main categories should be checked.

OXID eShop - gets first subcategory and redirect to this

<?php
/**
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * @copyright (c) Proud Sourcing GmbH | 2014
 * @link https://gist.github.com/proudcommerce
 * @link http://www.proudcommerce.com
 * @version 1.0.0
 **/
class glModules_alist extends glModules_alist_parent
{
    /**
     * main category oxids for redirect check
     * @var array
     */
    protected $_aRedirectMainCats = array('b24952903a91bc85a5378b2dfc72cf44', 'fa713d83c4cd5c14144e9fd05a91bc63', '1eacce546a769f8a9f1d21b5f0c5a848', '8f9682f1c9445fc17a447fa09e6cddc2', '1b5fcfb2cc0aa27c666cafdf8991aaae' );

    /**
     * Executes parent::render(), loads active category, prepares article
     * list sorting rules. According to category type loads list of
     * articles - regular (oxarticlelist::LoadCategoryArticles()) or price
     * dependent (oxarticlelist::LoadPriceArticles()). Generates page navigation data
     * such as previous/next window URL, number of available pages, generates
     * meta tags info (oxubase::_convertForMetaTags()) and returns name of
     * template to render. Also checks if actual pages count does not exceed real
     * articles page count. If yes - calls error_404_handler().
     *
     * @return  string  $this->_sThisTemplate   current template file name
     */
    public function render()
    {
        if (($oCategory = $this->getActiveCategory()))
        {
            if(in_array($oCategory->oxcategories__oxid->value, $this->_aRedirectMainCats))
            {
                if($aSubCats  = $oCategory->getSubCats())
                {
                    if ( is_array( $aSubCats ) )
                    {
                        foreach ( $aSubCats as $oSubCat )
                        {
                            oxRegistry::getUtils()->redirect( $oSubCat->getLink(), false, 302 );
                        }
                    }
                }
            }
        }
        return parent::render();
    }
}

to use redirect only, if no articles in category, change line 37 to:
if($this->getArticleList() == null && in_array($oCategory->oxcategories__oxid->value, $this->_aRedirectMainCats))