Don't fall thru to Hydra::View::NARInfo

This caused exceptions like:

  Caught exception in Hydra::Controller::Build->view_build "writing to file: Broken pipe at /nix/store/ihdb3widsq1dk7sbl5vqjxfcxb5ypad4-hydra-0.1pre1297-8158093/libexec/hydra/lib/Hydra/Controller/Build.pm line 59."

because the connection to the Nix daemon would be terminated due to a
protocol violation (calling queryPathInfo with an empty string).