fix not null value on refer id

inc/databap.php

@@ -270,7 +270,7 @@ class Databap extends PhpObject
 				'title' => "varchar(".self::TITLE_LEN.") NOT NULL",
 				'description' => "varchar(500) NOT NULL",
 				'link' => "varchar(200) NOT NULL",
-				'refer_id' => "int(10) UNSIGNED NOT NULL",
+				'refer_id' => "int(10) UNSIGNED",
 				'phrase' => "varchar(50) NOT NULL",
 				MySqlManager::getText(self::MSG_TABLE) => "varchar(500) NOT NULL",
 				'type' => "varchar(2) NOT NULL",