fix(ras-acc): intermittent auth + checkout flow error

[chickenn00dle]

All Submissions:

• [ ] Have you followed the Newspack Contributing guideline?
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Changes proposed in this Pull Request:

Closes https://app.asana.com/0/1207817176293825/1208379852549374

Fixes an issue where when modal checkout renders the cart with no errors, the modal is left in a loading state infinitely:

How to test the changes in this Pull Request:

This is very difficult to reproduce since the case of modal checkout rendering the cart page with no errors seems to only happen on the Hyperallergic beta test site, but we can mimic this by commenting out the following line of code: https://github.com/Automattic/newspack-blocks/blob/4bb284340a70750f9e2f65587510678424aeff93/includes/class-modal-checkout.php#L253

1. With the above line of code commented out, initiate the checkout modal via a checkout button.
2. Confirm you see a generic error with a back button appear in the modal:
3. Uncomment the code above back in, and smoke test the checkout modal to confirm nothing has broken

Other information:

• [ ] Have you added an explanation of what your changes do and why you'd like us to include them?
• [ ] Have you written new tests for your changes, as applicable?
• [ ] Have you successfully ran tests with your changes locally?