WPDB: Make sure we don't run sanity checks on DB dropins.

Previously, we'd run the sanity checks if `is_mysql` was not set to `false`. This caused problems for DB drop-ins that didn't define `is_mysql` at all. Instead, we can just check if `is_mysql` is `empty()`.

Also fix some unit tests that accidently ran correctly because of the strict `false ===` comparison.

Fixes #33501.



git-svn-id: https://develop.svn.wordpress.org/trunk@34655 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/includes/utils.php

@@ -380,6 +380,7 @@ class wpdb_exposed_methods_for_testing extends wpdb {
 		global $wpdb;
 		$this->dbh = $wpdb->dbh;
 		$this->use_mysqli = $wpdb->use_mysqli;
+		$this->is_mysql = $wpdb->is_mysql;
 		$this->ready = true;
 		$this->field_types = $wpdb->field_types;
 		$this->charset = $wpdb->charset;