Clarification on usage of kv_rpc.proto PutRequest expected_version

**QUESTION**

The kv_rpc.proto has a field [`expected_version`](https://github.com/apache/bookkeeper/blob/release-4.10.0/stream/proto/src/main/proto/kv_rpc.proto#L175) which I assume is for ensuring that the current version of a key is at the specified version prior to performing a change.

However there are still several open questions about how to properly use this functionality:
1. Is the version check functionality enabled whenever a value is provided here? Thus no value, no version checking?
2. If the above is indeed the case, how do you handle the version check when `version=0`? In protobuf, zero is the equivalent of a field being not provided. So there's no way to differentiate between checking for `version==0` and version checking disabled.
3. How do you do a check to ensure that the given key does *not* exist prior to a write? Can't use `version=0` as per the above 2 questions.

If we could get this added to the documentation on this field, it would be appreciated.