worker: fix nullptr deref after MessagePort deser failure

This would previously always have crashed when deserializing
a `MessagePort` fails, because there was always at least one
`nullptr` entry in the vector.

PR-URL: https://github.com/nodejs/node/pull/25076


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>