message: add the Update's empty constructor.
diff: @@ -145,6 +145,8 @@ pub trait Update {
  /// node has decided to rollback to a previous fork and un apply the
  /// given update.
  fn inverse(self) -> Self;
  +
  + fn empty() -> Self;
  }
  /// Define the Ledger side of the blockchain. This is not really on the blockchain
  