repl: do not run --eval code if there is none

`getOptionValue('--eval')` always returns a string, so it is never
loose-equal to `null`. Running eval makes some modifications to the
global object, including setting `module` to a different value, which
we want to avoid if possible.

Refs: https://github.com/nodejs/node/pull/27278
PR-URL: https://github.com/nodejs/node/pull/27587


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: John-David Dalton <john.david.dalton@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>