Fix: Handle empty string healthcheck as readiness healthcheck

[yangligt2]

What type of PR is this?
/kind bug
/kind failing-test

What this PR does / why we need it:
When High Availability (HA) is enabled on GKE, the GCP backend service health check uses an empty string service name, which incorrectly causes all pods to be reported as healthy, regardless of their actual readiness state.

This PR fixes this behavior by mapping the empty string health check to use the pod's readiness probe status. This ensures that the GCP backend service only considers ready pods as healthy and prevents traffic from being routed to pods that are not yet able to serve requests. This change does not affect the standard Kubernetes liveness and readiness probes.

Which issue(s) this PR fixes:
Fixes failing-test on GKE for #1337

Does this PR introduce a user-facing change?:
No

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[k8s-ci-robot]

Hi @yangligt2. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[kfswain]

very very minor, non-blocking nit: move the empty string case to the bottom of the list

[kfswain]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: kfswain, yangligt2

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• OWNERS [kfswain]

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[kfswain]

/ok-to-test

[kfswain]

/lgtm