Fix a bug causing Requsets_StackModel::findForRequests() to throw an exception.

application/modules/requests/models/StackModel.php

@@ -53,6 +53,9 @@ class Requests_StackModel extends Unl_Model
 
         $stacks = new Unl_Model_Collection(__CLASS__);
         foreach ($stackIds as $requestId => $stackId) {
+            if (!$stackId) {
+                continue;
+            }
             $stacks[$requestId] = Unl_Model_Registry::getInstance()->get(__CLASS__, $stackId);
         }
         return $stacks;