Fix/hyperallergic empty cart modal checkout

[chickenn00dle]

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• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
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Changes proposed in this Pull Request:

Small fixes for the changes in #1893

How to test the changes in this Pull Request:

See testing steps for #1893

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?

[chickenn00dle]

Sorry @miguelpeixe, thanks to @laurelfulford's digging I realized is_wc_endpoint_url doesn't actually check for the main WC cart and checkout page so had to make another change: b2b1786

Rerequesting review.