added test for empty-files successfully round-tripping back from runner
added test for empty-files successfully round-tripping back from runner

cyber-dojo/runner_stateless,cyber-dojo/runner_stateless,cyber-dojo/runner_stateless,cyber-dojo/runner_stateless,cyber-dojo/runner_stateless,cyber-dojo/runner,cyber-dojo/runner,cyber-dojo/runner_stateless,cyber-dojo/runner_stateless