EigenPod.verifyBalanceUpdate should handle the case where a validator's balance reaches 0 without having a "withdrawable epoch" set

[wadealexc]

Referencing discussions in the PR here: #238 (comment)

See also discussion on Slack: https://eigenlabs.slack.com/archives/C05PX94L3H9/p1697144597891359?thread_ts=1697139381.917899&cid=C05PX94L3H9

[Sidu28]

Had a discussion with @gpsanant about this today. We both agreed that there isn't a reasonable way to prove this edge case: when a Validator is slashed down to 0, is withdrawable but has not withdrawn

One course of action I suggested was that we leave this as is (its a very extreme edge case), document it well and make operators aware that this could happen. This really only becomes a problem when more than 33% of validators double sign and they all get slashed down to 0, which is nuts.

The more long term plan is that Im going to try to see if I can get Terence from Prysm's help to lobby the becaon chain devs to add a "withdrawn" field somewhere. Its a long shot but if we come up with a solid proposal, it might just work.