rpc: Allow empty JSON object value for `consensus_param_updates` in `/block_results` response

[ljoss17]

Closes: #1440

This PR adds a custom deserializer for the consensus_param_updates field in the /block_results response to deserialize both null and an empty JSON object to None.

E.g. both of these will successfully deserialize to None:

• {"consensus_param_updates": {}}

• {"consensus_param_updates": null}

• [x] Referenced an issue explaining the need for the change

• [ ] Updated all relevant documentation in docs

• [ ] Updated all code comments where relevant

• [ ] Wrote tests

• [x] Added entry in .changelog/

[romac]

Thanks! Can we also fix this while we are at it? https://github.com/informalsystems/tendermint-rs/blob/ed576f2f479e58fd269a2725c5b4730bc2a1776d/tendermint/src/consensus/params.rs#L19-L20

[ljoss17]

Thanks! Can we also fix this while we are at it?

https://github.com/informalsystems/tendermint-rs/blob/ed576f2f479e58fd269a2725c5b4730bc2a1776d/tendermint/src/consensus/params.rs#L19-L20

Oh I missed that sorry. I'll fix it