Remove lead slash

Addresses FriendsOfPHP/PHP-CS-Fixer#705
When there is no namespace detected it just skips the if-statement.
https://github.com/FriendsOfPHP/PHP-CS-Fixer/blob/master/Symfony/CS/Tokenizer/Tokens.php#L518-L531

I wonder if we should change this method to expect at least one namespace, or add a parameter to only return when the uses are in namespace(s).