NULL or DOMException

[equalsJeffH]

see w3c/webauthn#876 for full context.

as noted in w3c/webauthn#876 (comment), w3c/webauthn#876 (comment), w3c/webauthn#876 (comment), and w3c/webauthn#876 (comment), this is something to (eventually) deal with in credman.

Associated credman issue:

• create-a-cred and request-a-cred ought to return only a cred or error #129