Upgrade: Missing ADMIN_NAME_MINIMUM_LENGTH

The subject configuration setting, introduced in zc156, gets added to the site's configuration table on an initial install ... but not when a site's database is upgraded.

This results in a log similar to the following on a user-update in the admin:

[02-Aug-2019 17:41:38 UTC] Request URI: /myadmin/users.php, IP address: 47.49.36.106
#1  zen_update_user() called at [/home/my_site/mysite/myadmin/users.php:73]
--> PHP Notice: Use of undefined constant ADMIN_NAME_MINIMUM_LENGTH - assumed 'ADMIN_NAME_MINIMUM_LENGTH' in /home/my_site/mysite/myadmin/includes/functions/admin_access.php on line 192.

To correct, you'll need to copy/paste and then run the following SQL script in your admin's Tools->Install SQL Patches:

INSERT INTO configuration (configuration_title, configuration_key, configuration_value, val_function, configuration_description, configuration_group_id, sort_order, date_added) VALUES ('Admin Usernames', 'ADMIN_NAME_MINIMUM_LENGTH', '4', '{"error":"TEXT_MIN_ADMIN_USER_LENGTH","id":"FILTER_VALIDATE_INT","options":{"options":{"min_range":4}}}', 'Minimum length of admin usernames (must be 4 or more)', '2', '18', now());

Noting that the issue will be corrected in-core for zc157, based on this commit: https://github.com/zencart/zencart/commit/6a8efaa5f447e5955fcaec406f070d3b43de2689#diff-12584e0dd686c07b2fd119ca048fa362

Related issue: https://github.com/zencart/zencart/issues/2643