eth_getBlockByHash raises on missing block

[arnetheduck]

Per documentation, https://github.com/ethereum/wiki/wiki/json-rpc#eth_getblockbyhash returns a null object when the block cannot be found - a Option[Block..] would be a more natural mapping instead of a an exception with msg Parameter [result] expected JObject but got JNull - the latter is hard to filter for, and yet constitutes a "normal" non-exceptional response to the request.