repl: avoid crashing from null and undefined errors

When `throw undefined` or `throw null` is executed, the REPL crashes.
This change does a check for `null|undefined` before accessing an
error's properties to prevent crashing.

Fixes: https://github.com/nodejs/node/issues/16545
Fixes: https://github.com/nodejs/node/issues/16607

PR-URL: https://github.com/nodejs/node/pull/16574


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Lance Ball <lball@redhat.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>