Fix Puppet queries when no usable keys are available

The old behavior resulted in a call to `empty?` on a `Puppet::Util::Log` instance at https://github.com/crayfishx/hiera-gpg/blob/master/lib/hiera/backend/gpg_backend.rb#L38 when queried from puppet. This would throw a `MethodNotFound` exception, causing catalog compilation to fail completely rather than the expected result of a fallback to other hiera backends.