Should check for empty cells before sending request

[kevin1]

If I try to submit a csv with one of the uni cells blank, the request fails with:

Traceback (most recent call last):
  File "./canvas-wrangler.py", line 257, in <module>
    res = res.json()
  File "/usr/local/lib/python2.7/site-packages/requests/models.py", line 826, in json
    return complexjson.loads(self.text, **kwargs)
  File "/usr/local/Cellar/python/2.7.14/Frameworks/Python.framework/Versions/2.7/lib/python2.7/json/__init__.py", line 339, in loads
    return _default_decoder.decode(s)
  File "/usr/local/Cellar/python/2.7.14/Frameworks/Python.framework/Versions/2.7/lib/python2.7/json/decoder.py", line 367, in decode
    raise ValueError(errmsg("Extra data", s, end, len(s)))