Why is the "empty set of test" test not passing?

ConfigurationTest.php

<?php

use Symfony\Component\Config\Definition\Builder\TreeBuilder;
use Symfony\Component\Config\Definition\ConfigurationInterface;
use Symfony\Component\Config\Definition\Processor;

class ConfigurationTest extends \PHPUnit_Framework_TestCase
{
    /**
     * @dataProvider provideDefaultValues
     */
    public function testDefaultValues($settings)
    {
        $expected = array(
            'tests' => array(
                'default' => array(
                    'path' => 'Tests',
                ),
            ),
        );

        $config = $this->processConfig($settings);
        $this->assertEquals($expected, $config);
    }

    public function provideDefaultValues()
    {
        return array(
            'empty configuration' => array(
                array(),
            ),
            'empty set of test' => array(
                array(
                    'tests' => array(),
                ),
            ),
            'empty test' => array(
                array(
                    'tests' => array(
                        'default' => array(),
                    ),
                ),
            ),
        );
    }

    protected function getConfiguration()
    {
        return new Configuration();
    }

    protected function processConfig(array $config)
    {
        $configuration = $this->getConfiguration();
        $processor = new Processor();

        return $processor->processConfiguration($configuration, func_get_args());
    }
}

class Configuration implements ConfigurationInterface
{
    public function getConfigTreeBuilder()
    {
        $treeBuilder = new TreeBuilder();
        $rootNode = $treeBuilder->root('test_configuration');

        $rootNode
            ->children()
                ->arrayNode('tests')
                    ->fixXmlConfig('test')
                    ->addDefaultChildrenIfNoneSet('default')
                    ->useAttributeAsKey('name')
                    ->prototype('array')
                        ->children()
                            ->scalarNode('path')
                                ->defaultValue('Tests')
                            ->end()
                        ->end()
                    ->end()
                ->end()
            ->end()
        ;

        return $treeBuilder;
    }
}