Add a hotfix for missing DB_NAME constant

[JanJakes]

A hotfix for:

Fatal error: Uncaught Error: Undefined constant "DB_NAME" in /var/www/html/wp-content/mu-plugins/sqlite-database-integration/wp-includes/sqlite/db.php:64 Stack trace: #0 /var/www/html/wp-content/db.php(37): require_once() #1 /var/www/html/wp-includes/load.php(683): require_once('/var/www/html/w...') #2 /var/www/html/wp-settings.php(133): require_wp_db() #3 /var/www/html/wp-config.php(85): require_once('/var/www/html/w...') #4 /var/www/html/wp-load.php(50): require_once('/var/www/html/w...') #5 /var/www/html/wp-blog-header.php(13): require_once('/var/www/html/w...') #6 /var/www/html/index.php(17): require('/var/www/html/w...') #7 {main} thrown in /var/www/html/wp-content/mu-plugins/sqlite-database-integration/wp-includes/sqlite/db.php on line 64

This error occurs when importing a site backup in Studio that doesn't define the DB_NAME constant.

The ideal solution is to address this in Playground, but in the meantime, we can use a hotfix.

Fixes #40.