🐛 ensure etc client generators never return nil without an error

[fabriziopandini]

What this PR does / why we need it:
This PR is a backport to release 1-0 branch of #5710

Comment from original PR

This PR fixes etcd client generator panicking when etcd reports a leader-member, but the corresponding node does not exist in Kubernetes.
It introduces also a safeguard for another corner case (func called without any node)

[sbueringer]

/lgtm
based on that the change is identical to the one we merged into main

[CecileRobertMichon]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: CecileRobertMichon

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[fabriziopandini]

/cherry-pick release-0.4

[k8s-infra-cherrypick-robot]

@fabriziopandini: new pull request created: #5727

In response to this:

/cherry-pick release-0.4

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