webview js: Acknowledge nullable types on `getMessageNode` and friend.
webview js: Acknowledge nullable types on `getMessageNode` and friend.

Both `getMessageNode` and `getMessageIdFromNode` that uses it
can receive and handle maybe/nullable values for Node.

With stricter types it seems we sometimes use it as `undefined`.
So, to make the types more correct define them as `?Node`.

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile