[{ProductDetails:{"ProductId":1,Language:"fr"},Orders:[{"OrderId":11,"Amount":200},{"OrderId":12,"Amount":null}]},{ProductDetails:{"ProductId":2,Language:"es"},Orders:[{"OrderId":13,"Amount":300},{"OrderId":14,"Amount":400}]},{ProductDetails:{"ProductId":3,Language:"en"},Orders:[{"OrderId":15,"Amount":null}]}]

db.collection.update({"Orders.Amount":null,$expr:{$gt:[{$size:"$Orders"},1]}},{$pull:{Orders:{Amount:null}}},{multi:true})