Allow null along with undefined for pfx value.
This is to avoid breaking change when upgrading v14 to v16 and
3rd party library passing null to pfx
Fixes: https://github.com/nodejs/node/issues/36292
PR-URL: https://github.com/nodejs/node/pull/41170
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Filip Skokan <panva.ip@gmail.com>