stream: remove TODO and add a description instead

After looking into this it turned out that these two errors are
sanity checks that should not be reached. It is unfortunate that
we assigned error codes for these but changing it into an assertion
seems to be a hassle for `readable-streams`.

PR-URL: #27086
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Rich Trott <rtrott@gmail.com>

lib/_stream_transform.js

@@ -209,9 +209,7 @@ function done(stream, er, data) {
   if (data != null) // Single equals check for both `null` and `undefined`
     stream.push(data);
 
-  // TODO(BridgeAR): Write a test for these two error cases
-  // if there's nothing in the write buffer, then that means
-  // that nothing more will ever be provided
+  // These two error cases are coherence checks that can likely not be tested.
   if (stream._writableState.length)
     throw new ERR_TRANSFORM_WITH_LENGTH_0();