Proto: do not initialize the staking balance lazily

Context

In !10455 (merged), we keep track of the minimum of the delegated balance of a delegate over each cycle, but it is always zero in the cycle when a delegate is registered because of the lazy initialization of the staking balance.

This MR ensures that the staking balance is explicitly initialized when a delegate is registered. The lazy initialization was useless. Though we could think it was introduced at some point and some registered delegates may have no staking balance but it is probably wrong, and at least it is wrong on mainnet where I checked that the set of registered delegates and the keys of staking balances are the same.

This MR is a prerequisite for !10455 (merged)

Manually testing the MR

Checklist

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