Task: Revisit Missing email decission

When adding Apple SSO support, the API response also introduced support for returning the Sync email from the provided JWT token.

In the [current immplementation](https://github.com/Automattic/pocket-casts-ios/pull/409/files#diff-e1ee3e8d716e15b59667a3ed1ad5c97c7da85c0300ffe890847c97d48252289eR102-R104) (as of opening this task). We check for the email and avoid setting the value if the email is empty to prevent overriding a value set by the password flow and not returned by the API.

This check might not be needed when the changes for `user/login_apple` are merged to prod. We can revisit that decision and fail the login attempt within the common token request of [obtainToken](https://github.com/Automattic/pocket-casts-ios/pull/409/files#diff-032fe1f2cd3597f5dc28351d8a82ece28bea8ed8a091921206a45088eb1f5968L154).

Review the current email check in the implementation linked from PR #409 and the common token request in obtainToken. Confirm whether user/login_apple now guarantees an email in production; done means a login attempt fails there when the JWT email is missing instead of preserving the previous value.

Written by the indexing model from the issue text.