How to remove null and empty element in MongoDB?

Question

MongoDB

I'm trying to remove the null value and the empty { } element in reviews:

"reviews" : [
        {
                "author" : "Simon Holmes",
                "id" : ObjectId("57acdb16cec290c7a1466cf4"),
                "rating" : 5,
                "timestamp" : ISODate("2015-07-15T23:00:00Z"),
                "reviewText" : "What a great place. I can't say enough good things about it.",
                "_id" : ObjectId("57b6116a9d4d1e8bc7067387")
        },
        null,
        {

        }
]

Tried to delete via $unset

db.locations.update( {"name" : "Starcups 2"}, {$unset: { "reviews.1.null": ""  } } );