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/featured.php

@@ -30,7 +30,7 @@ class FeaturedAction extends StreamAction {
         $page = ($this->arg('page')) ? ($this->arg('page')+0) : 1;
 
         common_show_header(_('Featured users'),
-                           array($this, 'show_header'), NULL,
+                           array($this, 'show_header'), null,
                            array($this, 'show_top'));
 
         $this->show_notices($page);