Require DB_NAME to be set only in the new driver

[JanJakes]

Require DB_NAME to be set only in the new driver.

As reported in #194 (comment):

I have this plugin on a site, and the Plugins admin page offered me the upgrade. I upgraded, and it broke the site, due to previously I did not have define( 'DB_NAME', '...' ); in wp-config.php.

The error message was:

Error: Undefined constant "DB_NAME" in .../wp-content/plugins/sqlite-database-integration/wp-includes/sqlite/db.php on line 69

[adamziel]

In the new message let's explain it's about the DB_NAME constant and why do we need it in SQLite

[JanJakes]

@adamziel 👍 I improved the comment.