Automatic merge from submit-queue
Ensure slices are serialized as zero-length, not null
Fixes https://github.com/kubernetes/kubernetes/issues/43203 null serialization of slices to prevent NPE errors in clients that store and expect to receive non-null JSON values in these fields.
Ensures when we are converting to an external slice field that will be serialized even if empty (has `json` tag that does not include `omitempty`), we populate it with `[]`, not `nil`
Other places I considered putting this logic instead:
* When unmarshaling
* Would have to be done for both protobuf and ugorji
* Would still have to be done here (or on marshal) to handle cases where we construct objects to return
* When marshaling
* Would have to switch to use custom json marshaler (currently we use stdlib)
* When defaulting
* Defaulting isn't run on some fields, notably, pod template in rc/deployment spec
* Would still have to be done here (or on marshal) to handle cases where we construct objects to return
```release-note
API fields that previously serialized null arrays as `null` and empty arrays as `[]` no longer distinguish between those values and always output `[]` when serializing to JSON.
```
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