Fix absent namespaces for cluster-scoped resources in hierarchies

[nolar]

In #671, a case with cluster-scoped resources (which have no namespaces) was forgotten.

This PR fixes it: absent namespace in the owner object is not an error now, None is assigned as a namespace in children (as it was before the fix).

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Besides, a little optimization for clarity: the current contextual owner is looked up ONLY if namespace= is really unset, not when it is passed as None — for the case when adjust_namespace(…, namespace=None) is called from adopt() where the owner lookup is already performed.

The same distinguishing logic of None-vs-unset is applied for "name" harmonization — also a string that can be sometimes absent in the owner — at least, in theory.

The same for labels: passing None is a sign of an error (e.g., improper .get('labels')?): labels cannot be None, but they can be {} (as a default value in adopt()).

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There are almost no changes in the tests because the contracts/signatures/behaviour do not change (except for namespaces of cluster-scoped resources), only the implementation is changed.

fixes #725