Make accesstokens.user_agent NULL by default

The field is currently unused in the app, and it was causing problems in
SQLite.

Ref T529

schema.sql

@@ -15,7 +15,7 @@ CREATE TABLE IF NOT EXISTS `accesstokens` (
   `one_time` tinyint(1) NOT NULL DEFAULT '0',
   `created` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP,
   `expires` datetime DEFAULT NULL,
-  `user_agent` varchar(255) NOT NULL,
+  `user_agent` varchar(255) DEFAULT NULL,
   PRIMARY KEY (`token`)
 ) ENGINE=InnoDB DEFAULT CHARSET=latin1;
 

sqlite.sql

@@ -15,7 +15,7 @@ CREATE TABLE IF NOT EXISTS `accesstokens` (
   one_time INTEGER NOT NULL DEFAULT '0',
   created DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
   expires DATETIME DEFAULT NULL,
-  user_agent TEXT NOT NULL
+  user_agent TEXT DEFAULT NULL
 );
 
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