Allow ignoring results by running with `/dev/null` as paths

[schneems]

Currently this fails when using /dev/null in either:

        --on-success-dir <dir>       Success dir, relative to CWD. i.e. `<rundoc.md/dir>/rundoc_output/`.
        --on-failure-dir <dir>       Failure dir, relative to CWD i.e. `<rundoc.md/dir>/rundoc_failure/`.

It tries to create/delete the directory contents and that fails.