llv2 and rollv2 may report wrong reach if the aggregator crashes right before sending reach result to kingdom

[ple13]

Describe the bug
In the reach only and reach & frequency protocols, the aggregator first initializes the reach with 0L. It then runs an existOutputOr block to compute the next token. Inside this block, if the output blob is empty, it updates the reach and writes stuff to the output blob. Otherwise, it skips the block, and sends the reach to the kingdom.

There is a problem when the output blob exists, but the aggregator crashes before sending the reach to the kingdom. As the existOutputOr block is skipped, the reach will not be updated and will stay at the initialized value (0L). As a result, the kingdom will receive 0L.

Steps to reproduce
This is not reproducible as there are no tests to catch the possible issue yet.

Component(s) affected
Duchy (aggregator)

Version
NA

Environment
NA

Additional context
https://github.com/world-federation-of-advertisers/cross-media-measurement/blob/main/src/main/kotlin/org/wfanet/measurement/duchy/daemon/mill/liquidlegionsv2/LiquidLegionsV2Mill.kt#L704

https://github.com/world-federation-of-advertisers/cross-media-measurement/blob/main/src/main/kotlin/org/wfanet/measurement/duchy/daemon/mill/liquidlegionsv2/ReachOnlyLiquidLegionsV2Mill.kt#L596