RemoveItems converts empty maps and lists ({} and []) instead to null

I'm experiencing a similar issue as https://github.com/kubernetes-sigs/structured-merge-diff/pull/302 except for `RemoveItems()`

For example in my CronJob jobTempalte i end up with a metadata: null which is not a nullable field

```
"spec": {
"concurrencyPolicy": "Allow",
"failedJobsHistoryLimit": 1,
"jobTemplate": {
"metadata": null,
```
We should do the same as Extract items and preserve empty list/maps
```
# Starting value
mapOfMaps:
b:
a: "x"
c: "z"

# Call RemoveItems to remove all nested fields
RemoveItems(mapOfMaps.b.a, mapOfMaps.b.c)

# BUG: Returns null
mapOfMaps:
b: null
```

In my case the list/map is not empty before calling RemoveItems(). Only after calling RemoveItems() then the list/map is empty and returns null.

I think the correct behavior is we preserve the list or map if it becomes empty after using RemoveItems

Start by comparing RemoveItems with ExtractItems and the behavior described in pull request 302. Add regression coverage for removing all nested fields while preserving an empty map or list, then verify the result no longer becomes null in the shown mapOfMaps and CronJob cases.

Written by the indexing model from the issue text.