Fix for uncaught error at line 409 in notify.php

Hi,

In notify.php an error can occur at line 409.
Uncaught Error: Call to undefined method WC_Order_Refund::get_billing_first_name() if order is already refunded.

$order->get_billing_first_name() throws error if order is already refunded.

Tested Solution inserted after line 404 where you have a foreach( $orders as $order ) :

if ( is_a( $order, ‘WC_Order_Refund’ ) ) {
$order = wc_get_order( $order->get_parent_id() );
}

Please add this to the code for the updates, thanks!

See: https://github.com/woocommerce/woocommerce/issues/24531

Regards,
-Brian Bailie