LibJS: Convert matched regex result to string in Symbol.replace

This would crash on an undefined match (no match), since the matched
result was assumed to be a string (such as on discord.com). The spec
suggests converting it to a string as well:
https://tc39.es/ecma262/#sec-regexp.prototype-@@replace (14#c)

Userland/Libraries/LibJS/Tests/builtins/RegExp/RegExp.prototype.exec.js

@@ -148,3 +148,10 @@ test("empty character class semantics", () => {
     expect(res.length).toBe(1);
     expect(res[0]).toBe("x");
 });
+
+// #6409
+test("undefined match result", () => {
+    const r = /foo/;
+    r.exec = () => ({});
+    expect(r[Symbol.replace]()).toBe("undefined");
+});