atdgen: allow empty records 

It would be useful if empty records, as in
```
type t = {}
```
were accepted by `atdgen` and represented in OCaml as unit. One use case for this is when the record will, in the future, have fields added, but no fields are yet present. Given this is accepted by, e.g., `atdpy`, it would be useful if it were also accepted by other targets for uniformity.