Allow CORS Access-Control-Allow-Origin: null

[elizabeth-dev]

What this PR does / why we need it:

Right now, we are allowing to set the CORS header Access-Control-Allow-Origin through the annotation nginx.ingress.kubernetes.io/cors-allow-origin either to a valid set of origins, or to * (wildcard). However, the Fetch API standard that defines the CORS protocol specifies that a value null should be accepted as well. This PR adds this new lacking functionality along with its corresponding tests.

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 #8983

How Has This Been Tested?

Apart from adding its corresponding unit test, this has been tested with the following use-cases:

• Standalone null value in the cors-allow-origin annotation (happy path)
• Along with a different domain name origin (both origins work as expected)
• Between two different domain name origins in the annotation (all origins work as expected)
• Both a standalone domain name origin and a * wildcard (ensuring no breaking changes)

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]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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

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[k8s-ci-robot]

Hi @elizabeth-dev. Thanks for your PR.

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✅ Deploy Preview for kubernetes-ingress-nginx canceled.

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[longwuyuan]

/triage accepted
/kind bug
/ok-to-test