Proto/ai: don't update the Unstake_requests storage when not necessary

Context

Depends on !9406 (merged)

Only the last commit is in this MR

This MR optimizes the finalize_unstake call when no unstake request in unfinalizable. As I understand, the desired behavior was that prepare_finalize_unstake returned None if there was no finalizable requests, but this was not the case, so there were wasteful update calls. This MR should fix it. I don't think we can do much better than that in terms of performance, as it is necessary to scan all the unstake requests to know which ones are unfinalizable or not, if any.

Manually testing the MR

Checklist

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