Fixes 4123; Length check on originalFields of kustomizationFile to prevent panic

[m-Bilal]

Fixes #4123
kustomize edit was panicking when kustomization file began with a comment(or a blank line) which was immediately followed by a document separator because of len(mf.originalFields) was returning 0 here. More explanation in this comment.

cc: @KnVerey

[k8s-ci-robot]

Hi @m-Bilal. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[natasha41575]

/ok-to-test

[m-Bilal]

Is there anyway I can run the failing test on my system to check what's the problem here? make completes without any errors or failed tests when I run it, so I don't know which test might be failing and how these changes are responsible for the failing tests.

[natasha41575]

You can run make prow-presubmit-check locally to run all the presubmit checks.

The error looks kind of strange so I wonder if something weird happened in the CI.

[natasha41575]

/retest

[KnVerey]

Thanks for the detailed explanation of the problem and the fix! Makes sense to me.

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: KnVerey, m-Bilal

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