worker: fix process.env var empty key access

PR-URL: #32921
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Zeyu Yang <himself65@outlook.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Juan José Arboleda <soyjuanarbol@gmail.com>

src/node_env_var.cc

@@ -227,7 +227,7 @@ void MapKVStore::Set(Isolate* isolate, Local<String> key, Local<String> value) {
   Mutex::ScopedLock lock(mutex_);
   Utf8Value key_str(isolate, key);
   Utf8Value value_str(isolate, value);
-  if (*key_str != nullptr && *value_str != nullptr) {
+  if (*key_str != nullptr && key_str.length() > 0 && *value_str != nullptr) {
     map_[std::string(*key_str, key_str.length())] =
         std::string(*value_str, value_str.length());
   }