#2467: treat empty piraha cache file as if it was not there

Piraha has code to treat incomplete cache files (missing the <end> tag) as if they were not there. Pull request https://bitbucket.org/cactuscode/cactus/pull-requests/115/cactus-treat-existing-but-empty-piraha adds the same handling for existing but empty files. Currently this situation is handled via a die “Nothing”; instruction.