Make rspec-hiera-puppet compatible with Hiera 1.0.0

This change removes the call to Backend.empty_answer which was removed
from Hiera in this merge: https://github.com/puppetlabs/hiera/commit/f4411514847a13621c142b902cea2d8ed7197391

See also https://github.com/jkinred/hiera-gpg/commit/cc27a12455309b639fec990972dd186b3205c1c2
for the origin for this fix.