Fix Js.String.match_ return type

Javscript String.prototype.match function can
return undefineds for optional capture groups
that are not found, which breaks the type annotations
since this is not added as a Js.nullable, it's not
possible to deal with those undefined values since
it can't be passed to Js.Nullable.isNullable.

Author: renatoalencar

jscomp/others/js_string.ml

@@ -274,7 +274,7 @@ external localeCompare : t -> float = "localeCompare" [@@bs.send.pipe: t]
 ]}
 
 *)
-external match_ : Js_re.t -> t array option = "match" [@@bs.send.pipe: t] [@@bs.return {null_to_opt}]
+external match_ : Js_re.t -> t Js.nullable array option = "match" [@@bs.send.pipe: t] [@@bs.return {null_to_opt}]
 
 (** [normalize str] returns the normalized Unicode string using Normalization Form Canonical (NFC) Composition.