Merge pull request #573 from vasilvv/rate-absent

Make estimatedSendRate be `null` instead of `undefined`

index.bs

@@ -1420,8 +1420,8 @@ The dictionary SHALL have the following attributes:
    This rate applies to all streams and datagrams that share a [=WebTransport session=]
    and is calculated by the congestion control algorithm (potentially chosen by
    {{WebTransport/congestionControl}}). If the user agent does not
-   currently have an estimate, the member MUST be [=map/exist|absent=].
-   The member can be absent even if present in previous results.
+   currently have an estimate, the member MUST be the `null` value.
+   The member can be `null` even if it was not `null` in previous results.
 
 
 ## `WebTransportDatagramStats` Dictionary ##  {#web-transport-datagram-stats}