Use expected LaunchTemplateId in updating ASG when MixedInstancePolicy is changed

[h3poteto]

refs: #10718

When user changes only mixedInstancePolicy, LaunchTemplateId is not changed.
But current logic expect changes to LaunchTemplateId; therefore nil pointer dereference has occur.

In this case, it is not necessary to change LaunchTemplateId in AutoScalingGroup, so I fixed it.

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I don't know the reason for this logic. If anyone knows why, please tell me.

[k8s-ci-robot]

Hi @h3poteto. Thanks for your PR.

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[rifelpet]

/ok-to-test

[hakman]

Thanks @h3poteto!
/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: h3poteto, hakman

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