Default hiera_config to /dev/null instead of nil
Default hiera_config to /dev/null instead of nil

rodjek/rspec-puppet,rbramwell/rspec-puppet,glauco/rspec-puppet,iankronquist/rspec-puppet,jeremy-asher/rspec-puppet,johnmccabe/rspec-puppet,puppetlabs/rspec-puppet,electrical/rspec-puppet,tuxmea/rspec-puppet,fitpay/rspec-puppet,javiplx/rspec-puppet,camptocamp/rspec-puppet,DavidS/rspec-puppet