authpdo: fix mybb schema for MySQL 5.7+

https://dev.mysql.com/doc/refman/5.7/en/upgrading-from-previous-series.html

> In MySQL 5.7.5, these SQL mode changes were made: ... Strict SQL mode for transactional storage engines (`STRICT_TRANS_TABLES`) is now enabled by default.

https://dev.mysql.com/doc/refman/5.7/en/data-type-defaults.html

> For data entry into a NOT NULL column that has no explicit DEFAULT clause, if an INSERT or REPLACE statement includes no value for the column, or an UPDATE statement sets the column to NULL, MySQL handles the column according to the SQL mode in effect at the time:
>
> If strict SQL mode is enabled, an error occurs for transactional tables and the statement is rolled back. For nontransactional tables, an error occurs, but if this happens for the second or subsequent row of a multiple-row statement, the preceding rows will have been inserted.

lib/plugins/authpdo/_test/mysql/mybb.sql

@@ -30,13 +30,13 @@ CREATE TABLE `mybb_usergroups` (
   `gid` smallint(5) unsigned NOT NULL,
   `type` tinyint(1) unsigned NOT NULL DEFAULT '2',
   `title` varchar(120) NOT NULL DEFAULT '',
-  `description` text NOT NULL,
+  `description` text NOT NULL DEFAULT '',
   `namestyle` varchar(200) NOT NULL DEFAULT '{username}',
   `usertitle` varchar(120) NOT NULL DEFAULT '',
   `stars` smallint(4) unsigned NOT NULL DEFAULT '0',
   `starimage` varchar(120) NOT NULL DEFAULT '',
   `image` varchar(120) NOT NULL DEFAULT '',
-  `disporder` smallint(6) unsigned NOT NULL,
+  `disporder` smallint(6) unsigned NOT NULL DEFAULT '0',
   `isbannedgroup` tinyint(1) NOT NULL DEFAULT '0',
   `canview` tinyint(1) NOT NULL DEFAULT '0',
   `canviewthreads` tinyint(1) NOT NULL DEFAULT '0',
@@ -209,7 +209,7 @@ CREATE TABLE `mybb_users` (
   `google` varchar(75) NOT NULL DEFAULT '',
   `birthday` varchar(15) NOT NULL DEFAULT '',
   `birthdayprivacy` varchar(4) NOT NULL DEFAULT 'all',
-  `signature` text NOT NULL,
+  `signature` text NOT NULL DEFAULT '',
   `allownotices` tinyint(1) NOT NULL DEFAULT '0',
   `hideemail` tinyint(1) NOT NULL DEFAULT '0',
   `subscriptionmethod` tinyint(1) NOT NULL DEFAULT '0',
@@ -235,15 +235,15 @@ CREATE TABLE `mybb_users` (
   `timezone` varchar(5) NOT NULL DEFAULT '',
   `dst` tinyint(1) NOT NULL DEFAULT '0',
   `dstcorrection` tinyint(1) NOT NULL DEFAULT '0',
-  `buddylist` text NOT NULL,
-  `ignorelist` text NOT NULL,
+  `buddylist` text NOT NULL DEFAULT '',
+  `ignorelist` text NOT NULL DEFAULT '',
   `style` smallint(5) unsigned NOT NULL DEFAULT '0',
   `away` tinyint(1) NOT NULL DEFAULT '0',
   `awaydate` int(10) unsigned NOT NULL DEFAULT '0',
   `returndate` varchar(15) NOT NULL DEFAULT '',
   `awayreason` varchar(200) NOT NULL DEFAULT '',
-  `pmfolders` text NOT NULL,
-  `notepad` text NOT NULL,
+  `pmfolders` text NOT NULL DEFAULT '',
+  `notepad` text NOT NULL DEFAULT '',
   `referrer` int(10) unsigned NOT NULL DEFAULT '0',
   `referrals` int(10) unsigned NOT NULL DEFAULT '0',
   `reputation` int(11) NOT NULL DEFAULT '0',
@@ -264,7 +264,7 @@ CREATE TABLE `mybb_users` (
   `coppauser` tinyint(1) NOT NULL DEFAULT '0',
   `classicpostbit` tinyint(1) NOT NULL DEFAULT '0',
   `loginattempts` smallint(2) unsigned NOT NULL DEFAULT '1',
-  `usernotes` text NOT NULL,
+  `usernotes` text NOT NULL DEFAULT '',
   `sourceeditor` tinyint(1) NOT NULL DEFAULT '0'
 ) ENGINE=MyISAM AUTO_INCREMENT=88 DEFAULT CHARSET=utf8;