Check nameserver only when dns is enable

[yckaolalala]

What type of PR is this?

/kind bug

What this PR does / why we need it:

In PR #9502, Add a check for non-empty nameservers in /etc/resolv.conf for CoreDNS.
Should not check when dns_mode is set to manual or none.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Check nameserver only when dns is enable

[k8s-ci-robot]

Hi @yckaolalala. Thanks for your PR.

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[yankay]

Thanks @yckaolalala
/ok-to-test
/lgtm

[floryut]

@yckaolalala Thank you for the PR 👍

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: floryut, yckaolalala

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[yankay]

Thanks @yckaolalala
/lgtm