Unique indexes treat missing value as "unique" about fdb-document-layer HOT 2 CLOSED

> db.createCollection("tc")
{ "ok" : 1 }
> db.tc.insert({"a":1})
WriteResult({ "nInserted" : 1 })
> db.tc.insert({"a":2})
WriteResult({ "nInserted" : 1 })
> db.tc.find()
{ "_id" : ObjectId("5c37a995de954fb0637dec94"), "a" : 1 }
{ "_id" : ObjectId("5c37a997de954fb0637dec95"), "a" : 2 }
> db.tc.ensureIndex({"a":1}, unique=true)
{
	"createdCollectionAutomatically" : false,
	"numIndexesBefore" : 1,
	"numIndexesAfter" : 2,
	"ok" : 1
}
> db.tc.insert({"b":1})
WriteResult({ "nInserted" : 1 })
> db.tc.insert({"b":2})
WriteResult({
	"nInserted" : 0,
	"writeError" : {
		"code" : 11000,
		"errmsg" : "E11000 duplicate key error collection: test.tc index: a_1 dup key: { : null }"
	}
})