replace NULL with null

Another global search-and-replace update. Here, I've replaced the PHP
keyword 'NULL' with its lowercase version. This is another PEAR code
standards change.

darcs-hash:20081223192129-84dde-4a0182e0ec16a01ad88745ad3e08f7cb501aee0b.gz

actions/tagrss.php

@@ -41,7 +41,7 @@ class TagrssAction extends Rss10Action {
         $tag = $this->tag;
 
         if (is_null($tag)) {
-            return NULL;
+            return null;
         }
 
         $notice = Notice_tag::getStream($tag->tag, 0, ($limit == 0) ? NOTICES_PER_PAGE : $limit);