Immediately close the certsReady channel if the cert-generator is disabled

[tenzen-y]

What this PR does / why we need it:

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
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[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

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[tenzen-y]

In favor of #2209.
/close

[tenzen-y]

/close

[tenzen-y]

I'm not sure why the prow doesn't close this PR :(

[google-oss-prow]

@tenzen-y: Closed this PR.

In response to this:

/close

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[google-oss-prow]

@tenzen-y: Closed this PR.

In response to this:

In favor of #2209.
/close

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