rust: fix staking_params_are_empty

[ptrus]

oasis-core/runtime/src/consensus/registry.rs

Lines 292 to 294 in c31ac9d

	fn staking_params_are_empty(p: &RuntimeStakingParameters) -> bool {
	return Option::is_none(&p.thresholds);
	}

^ should be updated with new fields. Also add a test for consistent runtime struct serialization.

Originally posted by @kostko in #4455 (comment)