fix(tendermint): Make `FinalizeBlock::validator_updates` nullable when deserializing

[ec2]

Fixes: #1427

• [x] Referenced an issue explaining the need for the change
• [ ] Updated all relevant documentation in docs
• [ ] Updated all code comments where relevant
• [ ] Wrote tests
• [ ] Added entry in .changelog/

[romac]

Thanks for the PR! Can you please a changelog entry in .changelog/unreleased/improvements/?

[ec2]

@romac I've added the changelog :)