Fixing Stripe Checkout always overwriting tax value

[dparker1005]

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Changes proposed in this Pull Request:

When using Stripe Checkout, PMPro will always use the tax calculated by Stripe. This means that if a site is using the VAT Add On, the tax calculated by the Add On will be overwritten by the $0 tax calculated by Stripe.

This PR fixes that issue to only overwrite tax if Stripe is set up to calculate tax.

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Changelog entry

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[andrewlimaza]

I've retested this with the VAT Add On and it resolved the issue once marking the order as "Paid".