Make it possible to detect field changes when mixedInstancePolicy is removed

[h3poteto]

Refs: #11253

When I remove mixedInstancesPolicy using kops edit, there are difference actual and expected. But kops don't detect that changes. So, it was causing internal consistency error.
I changed buildChangeList, so kops can detect field changes when I remove mixedInstancePolicy.

[k8s-ci-robot]

Hi @h3poteto. Thanks for your PR.

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[hakman]

/ok-to-test

[johngmyers]

This brings up the issue of all those other places where we check a subfield of changes for nil.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: johngmyers

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