Permit [SameObject] for nullable types?

[jcjones]

WebAuthn made a previously-[SameObject] attribute userHandle nullable (in w3c/webauthn@3b2a1d1). @smaug---- points out that "SameObject applies to interface and objects only, not nullable types" but since this sort of makes sense, perhaps WebIDL should change to permit the behavior.

[annevk]

It either always returns null or always returns an object? In any event, given that we want to replace [SameObject] in #212 I'm not sure we should extend it now.

[jcjones]

Yeah, that's right. It's always either null, or a cached object, basically.