The DocumentKey field of the change event does not capture a shard key if the value of the shard key is null. 

Consider a collection emp with a shard key name.

Inserting a document with no value for name, like
{{db.emp.insertOne({"_id":"1001"}) 
}}produces a change event with DocumentKey that contains only the _id field
fullDocument=Document{{_id=1002, documentKey={"_id": "1001"}}}

Similarly, updating the name field does not have the previous value of the shard key in the document key. That is,
db.emp.updateOne({"_id":"1001","name":null},{$set:{"name":"Adam") }}also produces a change event with DocumentKey that contains only the _id field{{{}
documentKey={"_id": "1001"}, updateDescription=UpdateDescription{removedFields=[], updatedFields=

, truncatedArrays=[]}{}}}

In both these operations, the DocumentKey field should contain a field for the name as well, like documentKey={"_id": "1001", "name": null}

In our use case, we rely on the DocumentKey field to know the shard keys and their previous values. As a workaround we couldn't depend on retrieving the shard key from the config database when Change Streams is run as there is a possibility shard keys may have been refined between when the operation was performed and Change Streams was run.

Expected behavior:
Provide all the shard keys in DocumentKey even if their value is null.