Fix bug in Xml::_createChild Method

Hi !
===

Problem :
=======

Method : Xml::_createChild();

Example : 
```php
$xml = array(
	'tag' => array(
		'@' => 0,
		'@test' => 'A test'
	)
);
$obj = Xml::fromArray($xml);
$obj->asXML()
```

Output is :
```
<?xml version="1.0" encoding="UTF-8"?>
<tag test="A test"/>
```

That must be :
```
<?xml version="1.0" encoding="UTF-8"?>
<tag test="A test">0</tag>
```

Why ?
=====

```php
if ($childValue) {
	$child->appendChild($dom->createTextNode($childValue));
}
```
When `$childValue` equal "0", it's like `false`

Solution :
==========

Check if `$childValue` is null, because it's the default value of `$childValue`
```php
if (!is_null($childValue)) {
	$child->appendChild($dom->createTextNode($childValue));
}
```