allowed null and undefined

validators/string.ts

@@ -3,6 +3,7 @@ import { Validator, Args } from "../mod.ts";
 export const isString: Validator = {
   type: "isString",
   check: (value: any) => {
+    if (value === null || value === undefined) return;
     if (typeof value !== "string" && !(value instanceof String)) {
       return {};
     }