$eq: null works differently to mongodb

[forivall]

With MongoDB, $eq: null matches fields that are null and fields that are missing. For example, with the following mongosh operation

db.testcollection.find({ testvalue: null })

on a test db with the following document,

{
   "_id" : ObjectId("671206436e1e7e947a9b571e"),
   "otherwise_this_is_empty": true
}

then mongodb will return the document.

However, with UCAST (and downstream CASL), this document will not match, as testvalue will be undefined.

There's a few workarounds, like using $in: [undefined, null] or defining a custom compare function to buildMongoQueryMatcher (in CASL).

I have a test case prepared at forivall@1d1c5bb (branch)

If this is the intended behaviour, then I'd at least like the comparePrimitives method to be explicitly exported, so that the custom compare function can just be (a, b) => a == null && b == null ? 0 : comparePrimitives(a, b)