Don't break with DNS-PERSIST-01 challenges
The ACME spec now supports two additional authorization types:
- dns-persist-01
- dns-account-01
Both of these challenge types do not return any tokens (because they are validated with account level properties instead of order level properties), causing the client to break.
The dns-persist-01 challenge also returns a list of issuer-domain-names, which may be used to create the _validation-persist subdomains.