credentials.create should default to ES256 if options.pubKeyCredParams is empty

[nsatragno]

On [[Create]] method, step 9:

if credTypesAndPubKeyAlgs is empty and options.pubKeyCredParams is not empty, return a DOMException whose name is "NotSupportedError", and terminate this algorithm.

"NotSupportedError" should be returned if credTypesAndPubKeyAlgs is empty regardless of whether options.pubKeyCredParams is empty or not. This is because an empty list will fail when sent to authenticators anyway (step 2 of the authenticatorMakeCredential operation), so there is no point in sending the empty list.

At the moment chrome is immediately throwing NotSupportedError (which doesn't match the spec, it should wait for the user to tap an authenticator and return NotAllowedError instead) and firefox is defaulting to { type: "public-key", alg: -7 }

[nsatragno]

On the 2020-03-11 call, we discussed this issue and resolved to default to E256 if the list is empty. I'll upload a PR.