Avoid error if config.yml is empty

In ConfigSynchronizeCommand.php line 71:
                                                                               
  Flagbit\Shopware\ShopwareMaintenance\Command\ConfigSynchronizeCommand::exec  
  uteGlobalConfigSync(): Argument #1 ($yaml) must be of type array, null give  
  n, called in /app/vendor/flagbit/shopware-maintenance/Command/ConfigSynchro  
  nizeCommand.php on line 65