Added a failing test for #1.

In issue #1, the problem is that a directory with an empty string as
the relative path causes the pipeline to throw an error. Since all
instances of files with an empty relative path are directories and can't
be processed anyway, I suggested they be ignored.
Ignore null files.

Fixes #1.
Merge pull request #4 from jussi-kalliokoski/fix-issue-1

Fixing #1