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/logout.php

@@ -32,7 +32,7 @@ class LogoutAction extends Action {
         if (!common_logged_in()) {
             common_user_error(_('Not logged in.'));
         } else {
-            common_set_user(NULL);
+            common_set_user(null);
             common_real_login(false); # not logged in
             common_forgetme(); # don't log back in!
             common_redirect(common_local_url('public'));