Add back a database name fallback for Studio for now

WordPress Studio doesn't yet enforce the DB_NAME constant in "wp-config.php".
We'll use the same fallback value as Studio uses in the wp-now polyfill MU plugin.
This is now a safe change as the current connection database name is checked
against the actual database name of the existing database.

Author: JanJakes

wp-includes/sqlite/db.php

@@ -55,6 +55,20 @@
 require_once __DIR__ . '/class-wp-sqlite-db.php';
 require_once __DIR__ . '/install-functions.php';
 
+/**
+ * The DB_NAME constant is required by the SQLite driver.
+ *
+ * When DB_NAME is not defined, let's use a default value that corresponds to
+ * the default value of the constant in "wp-config-sample.php" and in Studio.
+ *
+ * TODO: Remove this once the DB_NAME constant is enforced in "wp-config.php".
+ */
+if ( defined( 'DB_NAME' ) && '' !== DB_NAME ) {
+	$db_name = DB_NAME;
+} else {
+	$db_name = 'database_name_here';
+}
+
 /*
  * Debug: Cross-check with MySQL.
  * This is for debugging purpose only and requires files
@@ -64,9 +78,9 @@
 $crosscheck_tests_file_path = dirname( __DIR__, 2 ) . '/tests/class-wp-sqlite-crosscheck-db.php';
 if ( defined( 'SQLITE_DEBUG_CROSSCHECK' ) && SQLITE_DEBUG_CROSSCHECK && file_exists( $crosscheck_tests_file_path ) ) {
 	require_once $crosscheck_tests_file_path;
-	$GLOBALS['wpdb'] = new WP_SQLite_Crosscheck_DB( DB_NAME );
+	$GLOBALS['wpdb'] = new WP_SQLite_Crosscheck_DB( $db_name );
 } else {
-	$GLOBALS['wpdb'] = new WP_SQLite_DB( defined( 'DB_NAME' ) ? DB_NAME : '' );
+	$GLOBALS['wpdb'] = new WP_SQLite_DB( $db_name );
 
 	// Boot the Query Monitor plugin if it is active.
 	require_once dirname( __DIR__, 2 ) . '/integrations/query-monitor/boot.php';