message: diagnostics writer now has a nullptr if we are not using it
diff: @@ -500,8 +500,7 @@ int command(int argc, const char *argv[]) {
  std::fstream::out),
  "# ");
  } else {
  - diagnostic_writers.emplace_back(
  - std::make_unique<std::fstream>("", std::fstream::out), "# ");
  + diagnostic_writers.emplace_back(nullptr, "# ");
  }
  }
  for (int i = 0; i < num_chains; i++) {
  