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/favoritesrss.php

@@ -25,7 +25,7 @@ require_once(INSTALLDIR.'/lib/rssaction.php');
 
 class FavoritesrssAction extends Rss10Action {
 
-    var $user = NULL;
+    var $user = null;
     
     function init() {
         $nickname = $this->trimmed('nickname');
@@ -68,6 +68,6 @@ class FavoritesrssAction extends Rss10Action {
     }
 
     function get_image() {
-        return NULL;
+        return null;
     }
 }