[Config] ArrayNode::addChild() - $node->getName() can't be 0

Node name (array key) cannot be 0

``` yaml
# bundle.yml
bundle:
    0: # won't work
        something: else
    1:
        this: works
```

``` php
$name = $node->getName();
if (empty($name)) { // problem is here
    throw new \InvalidArgumentException('Child nodes must be named.');
}
```

Easy fix will be something like `if (!strlen($name))`.
Thanks!
