Description:
------------
---
From manual pages:
* https://php.net/function.xmlwriter-write-attribute-ns
* https://www.php.net/manual/en/function.xmlwriter-write-element-ns.php
---

The URI element should be marked as nullable, see the stub:

https://github.com/php/php-src/blob/c05a9c3dcd587418906d9d25f83872e027f7dd43/ext/xmlwriter/xmlwriter.stub.php#L35

Test script:
---------------
<?php

$writer = new XMLWriter;
$writer->openMemory();
$writer->setIndent(true);
$writer->startElement('foo');
$writer->writeAttribute('xmlns:example', 'https://www.example.com/');
$writer->startElementNS('example', 'meta', null);
$writer->writeAttributeNS('example', 'spout', null, 'test');
$writer->endElement();
$writer->endElement();
echo $writer->outputMemory();