Fix Warnings.ml: use warnings <> [] instead of List.is_empty

Warnings.ml does not open Core, so List refers to the stdlib List
module (shadowed by -open Frontend). Use the simpler `warnings <> []`
pattern which works in any module context.

Author: ishaan-arora-1

src/frontend/Warnings.ml

@@ -13,5 +13,5 @@ let pp ?printed_filename ppf (span, message) =
   Fmt.pf ppf "@[<v4>%a%a%a@]@]" purple "Warning" maybe_loc span Fmt.text message
 
 let pp_warnings ?printed_filename ppf warnings =
-  if not (List.is_empty warnings) then
+  if warnings <> [] then
     Fmt.(pf ppf "@[<v>%a@]@\n" (list ~sep:cut (pp ?printed_filename)) warnings)