src: fix crypto.pbkdf2 callback error argument

Callbacks should always return `null` instead of `undefined` if no
error occurred.

PR-URL: nodejs#18458
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: BufoViridis

src/node_crypto.cc

@@ -4943,7 +4943,7 @@ void PBKDF2Request::Work(uv_work_t* work_req) {
 
 void PBKDF2Request::After(Local<Value> (*argv)[2]) {
   if (success_) {
-    (*argv)[0] = Undefined(env()->isolate());
+    (*argv)[0] = Null(env()->isolate());
     (*argv)[1] = Buffer::New(env(), key_, keylen_).ToLocalChecked();
     key_ = nullptr;
     keylen_ = 0;