crypto: use SetNull instead of Set

This commit updates the only usage of Set(Null(env->isolate()) to use
SetNull() instead which is used in other places in node_crypto.cc.

PR-URL: https://github.com/nodejs/node/pull/17521


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>