Do not return a '-1' exit if no keys found and json/yaml output

[hierynomus]

In our CI scripts we wanted to adapt to the new sshpublickey command, however for a new cluster this will exit non-successfully if there are no keys yet. It is possible to work around this with some ugly hacks, however for scripts I think we should return successfully if there are no public keys. It's not like something bad happened.

This PR ensures that for json/yaml output, which is for machine consumption, that even for an empty list it will exit with a 0 exit code if no errors happen.

[k8s-ci-robot]

Hi @hierynomus. Thanks for your PR.

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[hakman]

Seems like a good thing to me. Thanks @hierynomus.

[hakman]

/ok-to-test

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hakman

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[hierynomus]

/retest