[php-src] Issue #21583: SimpleXML: empty child element without attributes casts to true instead of false

Issue: https://github.com/php/php-src/issues/21583
Author: Chi-teck

### Description

The following code:

```php
<?php

$cases = [
  '<foo><bar>content</bar></foo>',
  '<foo><bar/></foo>',
  '<foo/>',
];
foreach ($cases as $case) {
  \printf("%-30s: %s\n", $case, new \SimpleXMLElement($case)->bar ? '✅' : '❌');
}
```

Resulted in this output:
```
<foo><bar>content</bar></foo>  : ✅
<foo><bar/></foo>              : ✅
<foo/>                         : ❌
```

But I expected this output instead:
```
<foo><bar>content</bar></foo>  : ✅
<foo><bar/></foo>              : ❌
<foo/>                         : ❌
```

The documentation explicitly lists empty SimpleXML elements without attributes as falsy.
https://www.php.net/manual/en/language.types.boolean.php#language.types.boolean.casting

<img width="1214" height="732" alt="Image" src="https://github.com/user-attachments/assets/e383dd74-5607-44e4-bbbe-17c49cf183f3" />

### PHP Version

```plain
any
```

### Operating System

Debian 13