[webauthn] excludeList can be defaulted to a zero-length sequence in IDL instead of prose

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

== excludeList can be defaulted to a zero-length sequence in IDL 
instead of prose ==
Instead of having 
https://w3c.github.io/webauthn/#dom-webauthentication-makecredential 
step 6 say

> If excludeList is undefined, set it to the empty list.

it would probably be better to have the IDL for the 
`ScopedCredentialOptions` dictionary say:

    sequence<ScopedCredentialDescriptor>  excludeList = [];

so you don't have to worry about this situation in prose at all.

Please view or discuss this issue at 
https://github.com/w3c/webauthn/issues/268 using your GitHub account