message: allow transaction without inputs or outputs
  the fee/balance will do the work
diff: @@ -134,12 +134,6 @@ impl<Extra: Clone> TransactionBuilder<Address, Extra> {
  fee_algorithm: F,
  policy: OutputPolicy,
  ) -> Result<(Balance, tx::Transaction<Address, Extra>), Error> {
  - if self.tx.inputs.len() == 0 {
  - return Err(Error::TxInvalidNoInput);
  - }
  - if self.tx.outputs.len() == 0 {
  - return Err(Error::TxInvalidNoOutput);
  - }
  // calculate initial fee, maybe we can fit it without any
  // additional calculations.
  let fee = fee_algorithm
  