Optimize non-deduplicated ISS to propose non-zero value if view change occurs

[ranchalp]

The current implementation of ISS' PBFT instances propose a default zero value (empty proposal) if there is a view change. This may have made sense where bucketization and deduplication of the request space occurred between PBFT instances, but with the current censorship-resistance, non-deduplicated version, allowing for a proposal of a non-empty value if the leader of the new view locally has availability certificates translates into more throughput at low cost.