fix: decode error on refund response

[potibm]

This PR fixes an unmarshaling error that happens when the Refund endpoint returns an empty {} body.

I just changed the code to decode into a standard json.RawMessage first. Now it works perfectly on my end.

As mentioned in the issue #308 , there are a few other types in checkouts.go and customers.go that might need a similar adjustment, but I only touched the Refund endpoint for now to get it working again.

[matoous]

Hi @potibm, thanks for the PR! I will close it in favor of: #310. The fix is correct, but we need to apply it in the codegen otherwise this would get overwritten the next time we generate the SDK from the OpenAPI specs.

[potibm]

Hi @matoous. Thanks for taking care! Should have taken a closer look at CONTRIBUTING.md :)