CurrencyResolverShippingTrait::calculateRates() crashes when order has no items (getTotalPrice() returns NULL)

Problem/Motivation

When manually creating or editing a draft Commerce order in the admin UI and adding a shipment before any order items exist, CurrencyResolverShippingTrait::calculateRates() throws a fatal error:

  Error: Call to a member function getCurrencyCode() on null in
  Drupal\commerce_currency_resolver_shipping\Plugin\Commerce\ShippingMethod\FlatRateCurrency->calculateRates()
  (line 45 of modules/shipping/src/Plugin/Commerce/ShippingMethod/CurrencyResolverShippingTrait.php)

This happens because $order->getTotalPrice() returns NULL when the order has no items, and the code calls ->getCurrencyCode() on it without a null check.

The same issue exists in selectRate() at line 68.

Steps to reproduce

1. Go to /admin/commerce/orders/add
2. Select a customer and store
3. Before adding any order items, click "Add shipment"
4. Enter a shipping address and submit
5. Fatal error on shipping rate calculation

Proposed resolution

Add null checks for getTotalPrice() in both calculateRates() and selectRate(), falling back to the parent implementation when the order has no total price yet.

public function calculateRates(ShipmentInterface $shipment): array {
    if ($order = $shipment->getOrder()) {
      $totalPrice = $order->getTotalPrice();
      if (!$totalPrice) {
        return parent::calculateRates($shipment);
      }
      $amount = $this->getRatesAmount($totalPrice->getCurrencyCode());
      // ... rest unchanged

public function selectRate(ShipmentInterface $shipment, ShippingRate $rate) {
    parent::selectRate($shipment, $rate);
    if ($order = $shipment->getOrder()) {
      $totalPrice = $order->getTotalPrice();
      if (!$totalPrice) {
        return;
      }
      $order_currency = $totalPrice->getCurrencyCode();
      // ... rest unchanged

This is consistent with how Commerce core addressed the same class of NULL price issues in #2998065: Ensure all calls to price getters handle NULL returns.