Support empty prefix in example for PSR-4

As many may use provided example of `Psr4AutoloaderClass` in their projects, this change adds support of empty prefix:
```
<?php
$loader->addNamespace('', '/path/to/packages/foo-bar/src');
// or
$loader->addNamespace('\\', '/path/to/packages/foo-bar/src');
```
The following line would cause the autoloader to attempt to load the `\Qux\Quux` class from `/path/to/packages/foo-bar/src/Qux/Quux.php`:
```
new \Qux\Quux;
```
The following line would cause the autoloader to attempt to load the `\Quux` class from `/path/to/packages/foo-bar/src/Quux.php`:
```
new \Quux;
```