Fixed non-existing index warning

git-svn-id: http://framework.zend.com/svn/framework/standard/trunk@20700 44c647ce-9c0f-0410-b52a-842ac1e357ba

library/Zend/Controller/Action/Helper/Cache.php

@@ -182,7 +182,7 @@ class Zend_Controller_Action_Helper_Cache
                 $tags = array_unique($this->_tags[$controller][$action]);
             }
             $extension = null;
-            if ($this->_extensions[$controller][$action]) {
+            if (isset($this->_extensions[$controller][$action])) {
                 $extension = $this->_extensions[$controller][$action];
             }
             $this->getCache(Zend_Cache_Manager::PAGECACHE)