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/public.php

@@ -31,7 +31,7 @@ class PublicAction extends StreamAction {
         header('X-XRDS-Location: '. common_local_url('publicxrds'));
 
         common_show_header(_('Public timeline'),
-                           array($this, 'show_header'), NULL,
+                           array($this, 'show_header'), null,
                            array($this, 'show_top'));
 
         # XXX: Public sidebar here?