Fix exception using ExternalName with empty endpoint list

[neerfri]

Due to kubernetes/kubernetes#114814 kubernetes no longer creates endpoints for services of type ExternalService. This in turn causes an exception when using an ingress that is backed by such service.

Due to #12952 , which I authored, this causes an exception reported in several recent issues that were opened after the new functionality was released.

I'm still puzzled why the e2e tests in https://github.com/kubernetes/ingress-nginx/blob/main/test/e2e/servicebackend/service_externalname.go did not catch those problems on effected versions of Kubernetes.

This is also related to:

• Regression when using ExternalName services #13081
• Regression in 1.11.5/1.20.1 when using ExternalName service with IP address #13076

What this PR does / why we need it:

We need this to stabilize the behavior of Ingress backend by a service with type ExternalName across different kubernetes versions and going forward.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• CVE Report (Scanner found CVE and adding report)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation only

Which issue/s this PR fixes

fixes #13081

How Has This Been Tested?

Adding tests to the Lua part of the code that was effected covering previously untested behavior and the specific exception from issue #13081

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have added unit and/or e2e tests to cover my changes.
• All new and existing tests passed.

[k8s-ci-robot]

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This pull-request has been approved by: neerfri
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[k8s-ci-robot]

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[k8s-ci-robot]

Hi @neerfri. Thanks for your PR.

I'm waiting for a kubernetes 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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Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

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