use NULL in database where possible

templates/posts.sql

@@ -1,9 +1,9 @@
 CREATE TABLE IF NOT EXISTS `posts_{{ board }}` (
   `id` int(11) NOT NULL AUTO_INCREMENT,
   `thread` int(11) DEFAULT NULL,
-  `subject` varchar(100) NOT NULL,
-  `email` varchar(30) NOT NULL,
-  `name` varchar(35) NOT NULL,
+  `subject` varchar(100) DEFAULT NULL,
+  `email` varchar(30) DEFAULT NULL,
+  `name` varchar(35) DEFAULT NULL,
   `trip` varchar(15) DEFAULT NULL,
   `capcode` varchar(50) DEFAULT NULL,
   `body` text NOT NULL,