Fix single table ordered queries.

Queries which contain only `sortBy` clause were not resolved as SingleTable expressions, so that following query would fail with exception:

```
query {
    for product in db.``[MAIN].[PRODUCTS]`` do
    sortBy product.PRODUCTNAME
    select { Product = { Name = product.PRODUCTNAME }
             Num = query { for _ in product.FK_OrderDetails_0 do count } }
} |> Seq.iter (fun x -> printfn "%s; %d" x.Product.Name x.Num)
```