Fixed case where config file might be empty

Fix for #136 

Minor bug fix, but when `phpspec.yml` is empty, trying to run PhpSpec results in a PHP Warning

> PHP Warning:  Invalid argument supplied for foreach() in [...]/phpspec/phpspec/src/PhpSpec/Console/Application.php on line 347

In `PhpSpec\Console\Application#loadConfigurationFile` the variable `$config` is initialized as an array.

After which `Yaml::parse` can return `null`, which it will if `phpspec.yml` is empty - and you can't iterate over null.