I have fixed it using below code, using JavaScript, writing fix details below, because I don't know Coffee Script to create pull-request, if fix is good in general too, then please help others via a pull request:

Fix details

in the file:

.\xml2js\lib\builder.js
OR
https://github.com/Leonidas-from-XIV/node-xml2js/blob/master/lib/builder.js

below code:

	} else if (Array.isArray(child)) {
		for (index in child) {

should be changed to:

	} else if (Array.isArray(child)) {

		// NOTE: FIX - begin
		if (child.length === 0) {
			element = render(element.ele(key), '').up();
		}
		// NOTE: FIX - end

		for (index in child) {

Oh, I didn't see your message. Well, I can only apply it when I have some spare time.

I'm not sure this needs a fix in xml2js. If you want an empty element in your XML, just add an empty string to your array:

let jsonObj = {
	key1: 'val2',
	arrayKey1: [456],
	emptyKey1: '',
	emptyArrayKey1: ['']
};

var builder = new xml2js.Builder();
var xml = builder.buildObject(jsonObj);
console.log(xml);

Output:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<root>
  <key1>val2</key1>
  <arrayKey1>456</arrayKey1>
  <emptyKey1/>
  <emptyArrayKey1/>
</root>