Address review: use List.is_empty with open Core

Author: ishaan-arora-1

src/frontend/Warnings.ml

@@ -1,3 +1,4 @@
+open Core
 module Location_span = Middle.Location_span
 module Location = Middle.Location
 
@@ -13,5 +14,5 @@ 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 warnings <> [] then
+  if not (List.is_empty warnings) then
     Fmt.(pf ppf "@[<v>%a@]@\n" (list ~sep:cut (pp ?printed_filename)) warnings)