Allow NULL values for value_old in changelog

db_schema.sql

@@ -24,7 +24,7 @@ CREATE TABLE IF NOT EXISTS `changelog` (
   `id_key` varchar(64) NOT NULL,
   `id_value` text NOT NULL,
   `field` varchar(255) NOT NULL,
-  `value_old` text NOT NULL,
+  `value_old` text,
   `value_new` text NOT NULL,
   `executed` text NOT NULL
 );