Problem
The server omits shard key fields in a change event's document key field if the document lacks that field. That is: for a collection sharded on (sk: 1, _id: 1), if a document is missing the sk field, its documentKey in the change stream is (_id: 1 ) rather than (sk: null, _id: 1). However, the server treats missing shard key fields as null internally.

Solution
Change event documentKeys should also include null shard key fields.

Impact
In WRITING-31304, the current logic will force mongosync to manually check for missing / null shard key fields based on the collection's shard key pattern. This is necessary to ensure that mongosync does queries on the full document key (instead of just the _id field). This is critical for mongosync correctness.

Fixing this logic will allow mongosync to eventually stop doing this workaround.