Allow to print creditmemos with no items

It is possible that creditmemos have no order items. Currently, if there are no order items, the following call to array_push will fail since it expects an array as first parameter:

    array_push($items['items'], array(
        'row_invoiced'     => $order->getShippingInvoiced(),
        'tax_inc_subtotal' => false,
        'tax_percent'      => $shippingTaxRate,
        'tax_amount'       => $shippingTaxAmount
    ));