Prune credit batch balance when no remaining balance

[ryanchristo]

Summary

Reported by @CShear and @blushi in slack. When an account has no remaining credits for a given credit batch, an empty balance is returned. To prevent empty balances from being included in the response when querying all credit batch balances, the empty balance should be pruned from state and therefore not included in query response.

Problem Definition

If users no longer hold credits for a given credit batch, the credit batch still shows up in the response when querying balances.

Proposal

Remove credit batch balance for a given account when no tradable, escrowed, or retired credits.

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