Consider where an empty sort spec is a valid use case and where it should be disallowed.

For example, here, empty sort specs are disallowed:

• SERVER-96579

  db.nodata.aggregate([{$group: {_id: "$a", o: {$top: {output: 'a', sortBy: {}}}}}]);
  MongoServerError[Location9657900]: Value for 'sortBy' must be a non-empty object
  

• rs0 [direct: primary] test> db.test.aggregate([{$match: {a: {$gte: 0}}}, {$sort: {}}])
  MongoServerError[Location15976]: $sort stage must have at least one sort key
  

Whereas here, they are a valid query:

• rs0 [direct: primary] test> db.test.find().sort({})
  [
    { _id: ObjectId('68790b382cd1e62966ffa76e'), a: 0, b: 0 },
    ...
  ]
  

The inspection should be done on a case-by-case basis.