Require DB_NAME to be set only in the new driver (#197)

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

Author: adamziel

wp-includes/sqlite/class-wp-sqlite-db.php

@@ -307,6 +307,14 @@ public function db_connect( $allow_bail = true ) {
 			$pdo = $GLOBALS['@pdo'];
 		}
 		if ( defined( 'WP_SQLITE_AST_DRIVER' ) && WP_SQLITE_AST_DRIVER ) {
+			if ( null === $this->dbname || '' === $this->dbname ) {
+				$this->bail(
+					'The database name was not set. The SQLite driver requires a database name to be set to emulate MySQL information schema tables.',
+					'db_connect_fail'
+				);
+				return false;
+			}
+
 			require_once __DIR__ . '/../../wp-includes/parser/class-wp-parser-grammar.php';
 			require_once __DIR__ . '/../../wp-includes/parser/class-wp-parser.php';
 			require_once __DIR__ . '/../../wp-includes/parser/class-wp-parser-node.php';

wp-includes/sqlite/db.php

@@ -66,5 +66,5 @@
 	require_once $crosscheck_tests_file_path;
 	$GLOBALS['wpdb'] = new WP_SQLite_Crosscheck_DB( DB_NAME );
 } else {
-	$GLOBALS['wpdb'] = new WP_SQLite_DB( DB_NAME );
+	$GLOBALS['wpdb'] = new WP_SQLite_DB( defined( 'DB_NAME' ) ? DB_NAME : '' );
 }