Change TRUE, FALSE, NULL capitalization to lowercase

lib/Models/Subscriber.php

@@ -162,7 +162,7 @@ class Subscriber extends Model {
       $subscriber = self::findOne($subscriber->id);
 
       // restore deleted subscriber
-      if($subscriber->deleted_at !== NULL) {
+      if($subscriber->deleted_at !== null) {
         $subscriber->setExpr('deleted_at', 'NULL');
       }
 
@@ -708,4 +708,4 @@ class Subscriber extends Model {
       )
     );
   }
-}
+}