// With insert the empty field is created.
> db.foo.insert({'': 'not allowed'})
WriteResult({ "nInserted" : 1 })
> db.foo.find()
{ "_id" : ObjectId("5314b690dd98b8b9decc41a7"), "" : "not allowed" }
> db.foo.drop()
true

// Replace-style upsert allows "" field names
> db.foo.find()
{ "_id" : ObjectId("5314b7b03a6a695da8595d60") }
> db.foo.drop()
true
> db.foo.update({}, {'': 'not allowed'}, {upsert:true})
WriteResult({
        "nMatched" : 0,
        "nUpserted" : 1,
        "nModified" : 0,
        "_id" : ObjectId("5314b7d03a6a695da8595d61")
})
> db.foo.find()
{ "_id" : ObjectId("5314b7d03a6a695da8595d61"), "" : "not allowed" }

// Other upserts don't allow empty fields:
> db.foo.drop()
true
> db.foo.update({'': 'not allowed'}, {$set:{a:1}}, {upsert:true})
WriteResult({
        "nMatched" : 0,
        "nUpserted" : 0,
        "nModified" : 0,
        "writeError" : {
                "code" : 56,
                "errmsg" : "An empty update path is not valid."
        }
})
> db.foo.drop()
false
> db.foo.update({}, {$set:{"":1}}, {upsert:true})
WriteResult({
        "nMatched" : 0,
        "nUpserted" : 0,
        "nModified" : 0,
        "writeError" : {
                "code" : 56,
                "errmsg" : "An empty update path is not valid."
        }
})