multitest returns error on BankMsg with 0 tokens

[ethanfrey]

This matches the actual behaviour of wasmd.

If you try BankMsg::Send{amount: vec![]} or BankMsg::Send{amount: coin(0, "foo")} you will get an error from the Cosmos SDK bank module. We should replicate this behaviour in the multitest system to detect such errors earlier on.

Check is amount.iter().filter(|x| !x.amount.is_zero()).len() == 0. In that case, return error