Fix outcome printing for empty objects (rescript-lang#444)

Author: amiralies

syntax/src/res_outcome_printer.ml

@@ -468,7 +468,7 @@ let printPolyVarIdent txt =
    and printObjectFields fields rest =
      let dots = match rest with
      | Some non_gen -> Doc.text ((if non_gen then "_" else "") ^ "..")
-     | None -> Doc.nil
+     | None -> if fields = [] then Doc.dot else Doc.nil
      in
      Doc.group (
        Doc.concat [

syntax/tests/oprint/expected/oprint.resi.txt

@@ -490,4 +490,5 @@ and ASet: {
   let find_last: (elt => bool, t) => elt
   let find_last_opt: (elt => bool, t) => option<elt>
   let of_list: list<elt> => t
-}
+}
+type emptyObject = {.}

syntax/tests/oprint/oprint.res

@@ -332,3 +332,5 @@ module rec A: {
     }
 }
 and ASet: Set.S with type elt = A.t = Set.Make(A)
+
+type emptyObject = {.}