[{"_id":1,"item":"ABC",price:80,"sizes":["S","M","L"]},{"_id":2,"item":"EFG",price:120,"sizes":[]},{"_id":3,"item":"IJK",price:160,"sizes":"M"},{"_id":4,"item":"LMN",price:10},{"_id":5,"item":"XYZ",price:5,"sizes":null}]

db.collection.aggregate([{$unwind:{path:"$sizes",preserveNullAndEmptyArrays:true}}])