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

scripts/fixup_notices_rendered.php

@@ -31,7 +31,7 @@ require_once(INSTALLDIR . '/lib/common.php');
 
 common_log(LOG_INFO, 'Starting to render old notices.');
 
-$start_at = ($argc > 1) ? $argv[1] : NULL;
+$start_at = ($argc > 1) ? $argv[1] : null;
 
 $notice = new Notice();
 if ($start_at) {