NULL == nullptr since C++ 11 so the function would return nullptr

At the callsite we'll check the truthiness of the return value

Can be more explicit and return nullptr?

Just kidding... Looks like the right thing to do is

return std::string{};

and check for empty string

if (blah.empty()) {
  return;
}

Alternatively I guess could return a std::optional<std::string>> or tuple of (grpc::Status, std::optional<std::string>) or something