broadcast_tx_commit response data property can be null

[sphw]

Some responses to broadcast_tx_commit will have a value of null for the data field in both check_tx and deliver_tx. Right now the fields aren't marked optional so it leads to an error being thrown by Serde. This could be solved in two ways. The data field could be marked as default and Serde would handle this, or the field could simply be an optional

[liamsi]

I really, really think we should use the proto files to generate the abci types instead of manually curate them (see discussions in #29)

[liamsi]

Oh, this actually about the rpc types. Here, we need better integration tests. It seems that these edge cases often aren't tested properly.