Fix TransferCommand::Combine CLI command (NoOutpoint error)

[zoedberg]

reporting an issue mentioned in #196:

The TransferCommand::Combine CLI command seems to have a bug: let blank_bundle = TransitionBundle::blank(&outpoint_map, &bmap! {})?; will always receive a NoOutpoint error, since the TransitionBundle::blank method expects to find at least an outpoint in it:

let (op, close_method) = new_outpoints
    .get(&input.ty)
    .ok_or(Error::NoOutpoint(input.ty))?;

So we cannot provide an empty bmap for the new_outpoints parameter.

As said in this comment #196 (comment) we should allow the specification of new outpoints via CLI, eventually failing back to the change outpoint if those are not provided