fix: expose empty labels

[mrueg]

What this PR does / why we need it:
To align with #2528
How does this change affect the cardinality of KSM: (increases, decreases or does not change cardinality)
Adds labels if they are 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 #

[richabanker]

/triage accepted
/cc @yongruilin
/assign @dgrisonnet

[k8s-ci-robot]

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In response to this:

/triage accepted
/cc @yongruilin
/assign @dgrisonnet

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[dgrisonnet]

/lgtm
/approve

[k8s-ci-robot]

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