Fix ECE crash when address has no defined shipping rates

[annemirasol]

Fixes #3538

Changes proposed in this Pull Request:

For block cart and block checkout, when Apple Pay, Google Pay or Link loads and the default address (first address, or last selected address) is not defined under Shipping, we get an IntegrationError and the cart/checkout page gets stuck trying to load the modal.

This happens because for block cart and block checkout, we pass a potentially empty shippingRates array option to ECE. When shippingRates is empty but shippingAddressRequired is true, the Stripe JS library throws an error and bails (Stripe docs).

This issue does not happen inside the Product and shortcode pages because we are always passing a non-empty "pending" shippingRates array when ECE loads.

To fix the issue, we will do the same for block cart and block checkout: when we do not have shipping rates for the current address, we will pass a "pending" shipping rates array. With this change, ECE will not crash the page, and the user has a chance to change to a valid shipping address, in case of multiple available addresses.

Testing instructions

1. Join the Google Pay test group to easily get multiple shipping addresses available to your account: https://groups.google.com/g/googlepay-test-mode-stub-data
2. As the merchant, set up your shipping zones such that only Japan is defined, with a free or flat rate shipping.
3. As the shopper, clear cookies and add a product to your cart.
4. For both block cart and block checkout, verify the following behavior:
   • On develop: Google Pay modal does not launch and the page is stuck loading.
   • On this branch:
     • Google Pay modal launches without any error.
     • Shopper still cannot proceed with the purchase if an address other than Japan is selected.
     • Shipping computation is correct when selecting a valid shipping address.
5. Regression test: the product, shortcode cart and shortcode checkout pages should not be affected and should behave the same as above.

Google Pay

Before	After

Apple Pay

Before	After

Link

Before	After

---

• Covered with tests (or have a good reason not to test in description ☝️)
• Added changelog entry in both changelog.txt and readme.txt (or does not apply)
• Tested on mobile (or does not apply)

Post merge

• Added testing instructions to the Release Testing Instructions wiki page (or does not apply)
• Added the needs docs label (or does not apply)
• Included this PR in the Release Thread scope (or does not apply)

[wjrosa]

I think it would be nice to have a simple unit test for this method.

[annemirasol]

Unit tests in c9c7efe, thanks!

[wjrosa]

The code is good, and it works as expected! Left a minor suggestion.

On develop: Google Pay modal does not launch and the page is stuck loading.

On this branch:
Google Pay modal launches without any error.

Shopper still cannot proceed with the purchase if an address other than Japan is selected.