Return a type predicate function that returns true if the type of x is T or undefined.

import { is } from "https://deno.land/x/unknownutil@v3.12.0/mod.ts";

const a: unknown = "a";
if (is.OptionalOf(is.String)(a)) {
 // a is narrowed to string | undefined
 const _: string | undefined = a;
}