[JS] Behaviour of Array.prototype.join() incompatible with ECMAScript v6.0

## Issue description
The implementation of method `Array.prototype.join()` diverts from the official [ECMAScript (ES) version 6.0](https://262.ecma-international.org/6.0/) documentation. ECMAScript treats `undefined` and `null` elements as empty strings, while this custom proprietary runtime first converts them to `null`, then converts `null` to string and only then treats these double converted strings as elements:
```js
// ECMAScript v6.0
var officialA = ["foo", undefined].join("/"); // -> "foo/"
var officialB = ["foo", null].join("/"); // -> "foo/"

// AppSync
var customA = ["foo", undefined].join("/"); // -> "foo/null"
var customB = ["foo", null].join("/"); // -> "foo/null"
```

Read the relevant section here - https://262.ecma-international.org/6.0/#sec-array.prototype.join
\- under the procedure step `13.c` you'll find stated:
> If _element_ is `undefined` or `null`, let _next_ be the empty String; otherwise, let _next_ be [ToString](https://262.ecma-international.org/6.0/#sec-tostring)(_element_).

## Bug reproduction
See the reproducible bug demo here: https://github.com/testuj-to/appsync-esv6-incompatible-join
In the demo, 2 strings are expected to be returned (`"foo/"` and `"foo/"`), instead this is the output:

Start with the linked reproducible demo and compare its Array.prototype.join() results with step 13.c of the ECMAScript 6.0 specification. Confirm that undefined and null elements should produce empty strings, then determine where an AppSync runtime report or change can be made; done means both examples return "foo/".

Written by the indexing model from the issue text.