Fix invalid default values in show create output

[jeroenpf]

This PR addresses an issue where the output of show create table is incorrect when the table has a NULL column without a default value.

Here is an example:

-- Original MySQL query

CREATE TABLE `wp_usermeta` (
	`umeta_id` bigint(20) unsigned NOT NULL auto_increment,
	`user_id` bigint(20) unsigned NOT NULL default '0',
	`meta_key` varchar(255) default NULL,
	`meta_value` longtext,
	PRIMARY KEY  (`umeta_id`),
	KEY `user_id` (`user_id`),
	KEY `meta_key` (`meta_key`(191))
)

-- Output by `SHOW CREATE TABLE` via SQLite plugin

CREATE TABLE `wp_usermeta` (
	`umeta_id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
	`user_id` bigint(20) unsigned NOT NULL DEFAULT '0',
	`meta_key` varchar(255) DEFAULT NULL,
	`meta_value` longtext DEFAULT , -- Default value missing
	PRIMARY KEY (`umeta_id`),
	KEY `meta_key` (`meta_key`),
	KEY `user_id` (`user_id`)
);

-- After this fix

CREATE TABLE `wp_usermeta` (
	`umeta_id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
	`user_id` bigint(20) unsigned NOT NULL DEFAULT '0',
	`meta_key` varchar(255) DEFAULT NULL,
	`meta_value` longtext,
	PRIMARY KEY (`umeta_id`),
	KEY `meta_key` (`meta_key`),
	KEY `user_id` (`user_id`)
);

[aristath]

Great catch!
Fix looks good 👍