message: chain-core: Comment on how to provide empty iterators
diff: @@ -120,9 +120,9 @@ pub trait Transaction: Serialize + Deserialize {
  type Input;
  /// The output type of the transaction (if none use `()`).
  type Output;
  - /// The iterable type of transaction inputs (if none use ??).
  + /// The iterable type of transaction inputs (if none use `Option<()>` and return `None`).
  type Inputs;
  - /// The iterable type of transaction outputs (if none use ??).
  + /// The iterable type of transaction outputs (if none use `Option<()>` and return `None`).
  type Outputs;
  /// a unique identifier of the transaction. For 2 different transactions
  /// we must have 2 different `Id` values.
  