RTCIceCandidate's relayProtocol and url members can be null but not absent

[jan-ivar]

The nullable readonly attribute relayProtocol says: "This is only present for local relay candidates." which is wrong since readonly attributes are always present, instead we should clarify when it is null.

Similarly, url says: "For local candidates of type "srflx" or type "relay" this is the URL of the ICE server from which the candidate was obtained." — add "Otherwise, it is null".

This seems minor and obvious, so marking it editorial.