Fix: Allow nullable card.country in Purchase type

[Devwill2]

Summary

The card.country field can be null in the database (no NOT NULL constraint) and the backend explicitly handles null values (purchase.rb:3704). Updated TypeScript type from string to string | null to match the actual data model and prevent runtime type validation errors.

Ai Discolsure

None. All code written by me

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As mentioned here : https://github.com/antiwork/gumroad/issues/1984#issuecomment-3594421927