🐛 Fix NPE when validating KCP etcd settings

[ncdc]

What this PR does / why we need it:
Fixes a panic when validating the etcd settings in KubeadmControlPlane if ClusterConfiguration is nil.

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 #2613

[ncdc]

/assign @chuckha

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ncdc

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[chuckha]

/lgtm

[k8s-ci-robot]

@ncdc: The following test failed, say /retest to rerun all failed tests:

Test name	Commit	Details	Rerun command
pull-cluster-api-capd-e2e	f2555be	link	/test pull-cluster-api-capd-e2e

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