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

Remove 'all namespaces' meaning of empty list in PodAffinityTerm

Removes the distinction between `null` and `[]` for the PodAffinityTerm#namespaces field (option 4 discussed in https://github.com/kubernetes/kubernetes/issues/43203#issuecomment-287237992), 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