repl: fix eval return value

In case no error has occurred during the evaluation of some code,
`undefined` has been returned in some cases as error argument instead
of `null`. This is fixed by this patch.

PR-URL: https://github.com/nodejs/node/pull/25731


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>