as discovered in https://gerrit.wikimedia.org/r/#/c/191512/ having an empty yaml file makes hiera lookup choke with this error:
Error: Could not retrieve catalog from remote server: Error 400 on SERVER: Error from DataBinding 'hiera' while looking up 'restbase::logging_level': undefined method `empty?' for nil:NilClass on node restbase1004.eqiad.wmnet