Creates an empty file.

def step_an_empty_file_named_filename(context, filename):
    """
    Creates an empty file.
    """
    assert not os.path.isabs(filename)
    command_util.ensure_workdir_exists(context)
    filename2 = os.path.join(context.workdir, filename)
    pathutil.create_textfile_with_contents(filename2, "")