Major Description
If you sort on field X, and it has no docvalues, today it will silently treat it as "all values missing". This can be very confusing since it just means nothing will happen at all.
But there is a distinction between "no docs happen to have a value for this field" and "field isn't configured correctly". The latter should get an exception, telling the user to index docvalues, or wrap the reader with UninvertingReader.