Extract Customer Details (email) for transactions not associated with a customer

[Kryt87]

In the SDK API the customer email stored with a Transaction could be extracted when no customer was associated with a transaction. This feature appears to be missing from the GraphQL API, where no email is returned if there is no customer associated with a transaction. Being unable to extract a transactions customer email using GraphQL prevents us from being able to migrate off of the SDK API because the email is required for fraud protection.