Produces a warning on uninhabitable sum type constructors

[Negabinary]

Here's an example in ReasonML:

open Ppx_yojson_conv_lib.Yojson_conv;

[@deriving yojson]
type empty =
  |;

[@deriving yojson]
type t =
  | A(int)
  | B(empty);

When I try to create a type with an uninhabitable case, the derived code causes the compiler to emit warning 56 : unreachable pattern. I also can't work out if there's a way to disable warning 56 on just the generate code here.

(I'm also seeing this on the sexp deriver janestreet/ppx_sexp_conv#44)