🐛 fix structuredmerge filterintent to cleanup nested empty maps

[chrischdi]

What this PR does / why we need it:

FilterIntent is supposed to remove all empty maps including nested maps to get consistent objects for comparison.

Currently it did only remove empty leaf maps which exist due to field removal.
Now it also removes new empty leaf maps which result from removing an empty leaf map.

Example:

• Consider the following input

  map[string]interface{}{
    "a": map[string]interface{}{
      "b": map[string]interface{}{
        "c": map[string]interface{}{
          "d": "123",
        },
      },
    },
  }

• Consider FilterIntent to remove the path a.b.c.d (due to ignorePath).

• The current implementation would only remove the resulting empty leaf map at a.b.c which would result in:

  map[string]interface{}{
    "a": map[string]interface{}{
      "b": map[string]interface{}{
      },
    },
  }

• Actually FilterIntent should also remove a.b and then a because while bubbling up they also get empty leaf maps.

• The result including this fix is:

  map[string]interface{}{
  }

/test all

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

[sbueringer]

/cherry-pick release-1.2

[k8s-infra-cherrypick-robot]

@sbueringer: once the present PR merges, I will cherry-pick it on top of release-1.2 in a new PR and assign it to you.

In response to this:

/cherry-pick release-1.2

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[sbueringer]

/lgtm

/cc @ykakarap @killianmuldoon

[sbueringer]

/approve

[k8s-ci-robot]

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[k8s-infra-cherrypick-robot]

@sbueringer: new pull request created: #6796

In response to this:

/cherry-pick release-1.2

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.