This first block in Tendermint is unique in that all of the last_xxx fields are actually empty, while in normal operation they are required to not be empty. On the other hand, some fields can be empty in normal operation (eg. app_hash, data_hash, evidence_hash). Recent PRs like #66 have been dealing with this by making all fields that can be empty Options, but it may be clearer if there is actually an explicit FirstBlock type that doesn't have the empty fields and and we reserve Option only for the fields that can be empty in normal operation. See the discussion in #66.

Another possibility is to clean this up by making a chance in the underlying tendermint protocol, see tendermint/tendermint#4241