404: Pass correct get_site_url param to avoid infinite loop (#98)

The first param to `get_site_url()` should be a site ID, or `null` for the current site. Passing a string can cause a `switch_to_blog( 0 )` in some circumstances. That breaks SQL queries because the table doesn't exist. The first broken query results in a `wp_die()`, but that leads to more queries which in turn call `wp_die()`.

Passing `null` fixes the error while preserving the original intent from #54 (comment).

source/wp-content/themes/wporg-parent-2021/patterns/404-page-subtitle.php

@@ -13,7 +13,7 @@
 	printf(
 		/* translators: %s is the site URL. */
 		wp_kses_post( __( 'Go to <a href="%s">the homepage</a> or try searching using the field below.', 'wporg' ) ),
-		esc_url( get_site_url( '/' ) )
+		esc_url( get_site_url( null, '/' ) )
 	);
 	?>
 </p>