message: improve confusing debug msg when empty keys
diff: @@ -194,7 +194,7 @@ where
  .map(|key: &ObjectId| key.to_string())
  .collect::<Vec<_>>()
  .join(", ");
  - debug!("Initialized with: {}", keys);
  + debug!("Initialized with: [{}]", keys);
  Ok((data, version))
  }
  