Don't return empty vector from raw_xxx_dn_sha256 if SHA-256 disabled
Don't return empty vector from raw_xxx_dn_sha256 if SHA-256 disabled

This can't happen currently because x509 module has a hard dep
on SHA-256 but if that changed in the future it would expose
incorrect behavior.

randombit/botan,randombit/botan,randombit/botan,randombit/botan,randombit/botan