[webauthn] FIDO U2F Attestation Statement Format doesn't say what to do with AAGUID

jcjones has just created a new issue for https://github.com/w3c/webauthn:

== FIDO U2F Attestation Statement Format doesn't say what to do with AAGUID ==
The AAGUID field is a required part of the Authenticator Data structure. U2F devices may not have an AAGUID. (I think, if it exists, it's in the attestation certificate?)

We should define what the 128-bit AAGUID should be if if isn't known, instead of leaving it implementation-specific. Right now Firefox is leaving it as all zeroes (and not even trying to pull it out of the certificate -- that's a TODO), but we'll swap to whatever is appropriate.

Please view or discuss this issue at https://github.com/w3c/webauthn/issues/506 using your GitHub account