Fix noop granular case for atomic<->granular schema changes

[jpbetz]

If the managed fields for a complex type has no children fields (e.g. emptyrDir: {}), and the live object has not fields, skip reconciliation with schemas entirely.

Fixes the integration test failure in k8s: https://prow.k8s.io/view/gs/kubernetes-jenkins/pr-logs/pull/93901/pull-kubernetes-integration/1311377692632813568. How I reproduced:

kubectl apply -f hack/testdata/retainKeys/deployment/deployment-before.yaml
kubectl apply -f hack/testdata/retainKeys/deployment/deployment-after.yaml
kubectl get deployment test-deployment-retainkeys -oyaml | grep "emptyDir"

Before this fix, emptyDir: {} would match in the fieldset because reconciliation created a replacement fieldset for the emptyDir that has both a Member and an empty Children list (created as a side effect of calling descendToPath), which prevented it from being properly removed when deployment-after.yaml was applied and the emptyDir was removed entirely and replaced with a hostPath.

[jpbetz]

@apelisse One last fix and then I think k8s tests should all pass.

[apelisse]

shrug
/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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