worker: do not crash when JSTransferable lists untransferable value

This can currently be triggered when posting a closing FileHandle.

Refs: https://github.com/nodejs/node/pull/34746#issuecomment-673675333

PR-URL: https://github.com/nodejs/node/pull/34766
Backport-PR-URL: https://github.com/nodejs/node/pull/34814


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>