Initialize className variable with an empty string (#2002)

Author: elidrissidev

app/code/core/Mage/Core/Model/Config.php

@@ -1271,7 +1271,7 @@ public function getGroupedClassName($groupType, $classId, $groupRootNode = null)
         $config = $this->_xml->global->{$groupType.'s'}->{$group};
 
         // First - check maybe the entity class was rewritten
-        $className = null;
+        $className = '';
         if (isset($config->rewrite->$class)) {
             $className = (string)$config->rewrite->$class;
         } else {