Dataset Bundle Error when no README

[bbengfort]

If there is no README the celery task dataset.tasks.bundle_dataset_version raises an exception:

[2016-12-13 21:00:40,456: ERROR/MainProcess] Task dataset.tasks.bundle_dataset_version[1f86cc63-f4d8-4e3c-8874-683e6890fa47] raised unexpected: TypeError("object of type 'NoneType' has no len()",)
Traceback (most recent call last):
  File "/Users/benjamin/.virtualenvs/cultivar/lib/python3.5/site-packages/celery/app/trace.py", line 240, in trace_task
    R = retval = fun(*args, **kwargs)
  File "/Users/benjamin/.virtualenvs/cultivar/lib/python3.5/site-packages/celery/app/trace.py", line 438, in __protected_call__
    return self.run(*args, **kwargs)
  File "/Users/benjamin/Repos/ddl/cultivar/dataset/tasks.py", line 44, in bundle_dataset_version
    version.bundle()
  File "/Users/benjamin/Repos/ddl/cultivar/dataset/models.py", line 197, in bundle
    self._zip(self.temp_directory)
  File "/Users/benjamin/Repos/ddl/cultivar/dataset/models.py", line 218, in _zip
    zf.writestr('readme.md', self.dataset.readme.raw)
  File "/usr/local/Cellar/python3/3.5.1/Frameworks/Python.framework/Versions/3.5/lib/python3.5/zipfile.py", line 1552, in writestr
    zinfo.file_size = len(data)            # Uncompressed size
TypeError: object of type 'NoneType' has no len()