[php-src] Issue #21077: Accessing Dom\Node::baseURI can throw TypeError

Issue: https://github.com/php/php-src/issues/21077
Author: mbeccati

### Description

The following code:

```php
<?php
$implementation = new \Dom\Implementation();
$node = $implementation->createDocumentType('html', 'publicId', 'systemId');

var_dump($node->baseURI);
```

Resulted in this output:
```
Fatal error: Uncaught TypeError: Cannot assign null to property Dom\Node::$baseURI of type string in ext/dom/tests/node_baseuri_null.php:6
```

But I expected this output instead:
```
NULL
```

Found running the Symfony test suite, failure in PHP 8.4+. Implementation is capable of generating a NULL baseURI, which isn't allowed in the stub, even if the property is expected to be nullable.
```
1) Symfony\Component\VarDumper\Tests\Caster\DOMCasterTest::testCastModernDocumentType
TypeError: Cannot assign null to property Dom\Node::$baseURI of type string

/home/runner/work/php-latest-builds/php-latest-builds/src/Symfony/Component/VarDumper/Caster/DOMCaster.php:190
```

### PHP Version

```plain
PHP 8.4.19-dev (cli) (built: Jan 29 2026 11:56:43) (NTS DEBUG)
Copyright (c) The PHP Group
Zend Engine v4.4.19-dev, Copyright (c) Zend Technologies
```

### Operating System

Irrelevant