(delta/sds): Mark secret as ready if server says the resource doesn't exist

[keithmattix]

As discussed in Slack, a delta removal should complete cluster warming

Commit Message: (fix/sds): Mark cluster as ready after
Additional Description:
Risk Level: Low
Testing: integration
Docs Changes:
Release Notes:
Platform Specific Features:
[Optional Runtime guard:]
[Optional Fixes #Issue]
[Optional Fixes commit #PR or SHA]
[Optional Deprecated:]
[Optional API Considerations:]

[adisuissa]

Small nits but otherwise LGTM, thanks!
Assigning Harvey for senior-maintainer review.
/assign @htuch

[htuch]

Does this need runtime protection? Seems pretty minor corner case, so I'm inclined to say no, but just putting it out there.

[keithmattix]

Does this need runtime protection? Seems pretty minor corner case, so I'm inclined to say no, but just putting it out there.

Runtime protection in what sense?

[htuch]

As per https://github.com/envoyproxy/envoy/blob/main/CONTRIBUTING.md#breaking-change-policy. I think it's fine TBH in this case, but for next behavior change it's worth familiarising.

[htuch]

Can you update the version history to be clear this changed?

[keithmattix]

IIUC, when you take both of my PRs together, the only change to the user is that an SDS removal of a secret that doesn't exist yet is NACK'd instead of ACK'd. Based on the breaking change policy (thanks for the link!), the first PR was the breaking change but this PR reverts to the status quo with respect to the init target being marked as ready. Sorry if I'm misunderstanding something

[htuch]

OK, makes sense, thanks.