$setWindowFields implements partitioning by sorting. We should make sure partitioning works when the partition key is a mixture of nulls and missing.

Should we coerce missing to null? ($group does, to ensure the _id field is always present.) At minimum, we need to make sure that all nulls are in the same partition, and all missings are in the same partition.

We should also make sure to test compound keys where the components are a mixture of null and missing.