When mapping is not defined then clear cache fails

This modification allowes to clear cache in symfony when no mapping is defined.

Error during clear-cache:



[ErrorException]                                                                                                                                                                                     
  Warning: Invalid argument supplied for foreach() in //symfony-standard/vendor/exercise/elastica-bundle/FOQ/ElasticaBundle/DependencyInjection/Configuration.php line 375  
                                                                                                                                                                                                       


Script Sensio\Bundle\DistributionBundle\Composer\ScriptHandler::clearCache handling the post-update-cmd event terminated with an exception


                                                                             
  [RuntimeException]                                                         
  An error occurred when executing the "'cache:clear --no-warmup'" command.