📖 CRS API does not allow empty selectors

[Madhur97]

What this PR does / why we need it:
Updated CRS API and documentation that label selector can not be empty.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #4635

[k8s-ci-robot]

Hi @Madhur97. Thanks for your PR.

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[fejta-bot]

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[fabriziopandini]

@Madhur97 thanks for this PR!
Please sign CLA
/ok-to-test

[fabriziopandini]

After modifying API comments, it is also necessary to run make generate-manifests

[fabriziopandini]

/retest
/lgtm

[vincepri]

/approve

[k8s-ci-robot]

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