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: #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>

Author: danbev

src/node_crypto.cc

@@ -1595,7 +1595,7 @@ void SecureContext::GetCertificate(const FunctionCallbackInfo<Value>& args) {
   else
     cert = wrap->issuer_;
   if (cert == nullptr)
-    return args.GetReturnValue().Set(Null(env->isolate()));
+    return args.GetReturnValue().SetNull();
 
   int size = i2d_X509(cert, nullptr);
   Local<Object> buff = Buffer::New(env, size).ToLocalChecked();