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

lib/deleteaction.php

@@ -43,7 +43,7 @@ class DeleteAction extends Action {
         }
     }
 
-    function show_top($arr=NULL) {
+    function show_top($arr=null) {
         $instr = $this->get_instructions();
         $output = common_markup_to_html($instr);
         common_element_start('div', 'instructions');
@@ -52,7 +52,7 @@ class DeleteAction extends Action {
     }
 
     function get_title() {
-        return NULL;
+        return null;
     }
 
     function show_header() {