Remove 'all namespaces' meaning of empty list in PodAffinityTerm

[liggitt]

Removes the distinction between null and [] for the PodAffinityTerm#namespaces field (option 4 discussed in #43203 (comment)), since we can't distinguish between them in protobuf (and it's a less than ideal API)

Leaves the door open to reintroducing "all namespaces" function via a dedicated field or a dedicated token in the list of namespaces

Wanted to get a PR open and tests green in case we went with this option.

Not sure what doc/release-note is needed if the "all namespaces" function is not present in 1.6

[k8s-reviewable]

This change is 

[liggitt]

@davidopp @aveshagarwal PTAL
cc @kubernetes/sig-scheduling-misc

[davidopp]

I hadn't seen you PR, so sent out #43278 to add the "*" behavior. (But see the comment about my build environment being messed up so couldn't run hack/update-all.sh or test it.)

I think it's slightly better if we can do the "*" at the same time as changing the nil/empty behavior, but if nobody has time to pick up that PR then I'm OK with this one.

[aveshagarwal]

@liggitt thanks lgtm.

[derekwaynecarr]

@davidopp -- i would like @kubernetes/sig-scheduling-misc to start thinking about a PodSchedulingPolicy object that I could use to provide more granular control where its needed. So if I do not want to let pods in namespace A stalk pods in namespace B, I should be able to control that via policy. For example, I do not want to let users stalk any of the kube-system components (scheduler, etc.) unless I entitle them to do that action.

[davidopp]

/lgtm

[davidopp]

/approve

[k8s-github-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: davidopp, liggitt
We suggest the following additional approver: @lavalamp

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[davidopp]

I'm marking this "release-note-none" and going to update the manual release notes to mention this.

[davidopp]

@k8s-bot verify test this

[k8s-github-robot]

Automatic merge from submit-queue (batch tested with PRs 43313, 43257, 43271, 43307)