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/finishaddopenid.php

@@ -97,7 +97,7 @@ class FinishaddopenidAction extends Action {
 
     function message($msg) {
         common_show_header(_('OpenID Login'));
-        common_element('p', NULL, $msg);
+        common_element('p', null, $msg);
         common_show_footer();
     }
 }