domain_exists should add trailing slash to path

[ejdanderson]

insert_blog runs trailingslashit on the path, however, domain_exists uses whatever is passed in. If WP is enforcing the trailing slash on blog insertion, it really should when checking existence. It should probably also cast the site_id as an int per insert_blog.

When working on #21142, I encountered $path being empty when creating a site via wp-signup.php, so I agree that we should be running trailingslashit on the path.

The $site_id parameter does not need to be cast to an integer though, because the $wpdb->prepare() takes care of that.