Module PS4 : $namespace of constructor must be a string

Actually, by default it is null :
```php
	public function __construct($baseDir, $namespace = null) {
```
but it must be a string because of trim function :
```php
		$this->namespace = trim($namespace, '\\');
```

So constructor must be defined by :
```php
	public function __construct($baseDir, $namespace = '') {
```
