Filter 0-balance accounts from staking ledgers

[psteckler]

Almost 3/4 of accounts have zero balance, and these don't contribute to the stake seen by block producers. Filtering these would save a lot of space.

[psteckler]

That changes the ledger hash, so maybe not a good idea.

[psteckler]

But also: https://github.com/MinaProtocol/mina/issues/9393

[deepthiskumar]

Impact: Reduces disk usage