handle empty values in iarc xml resp (bug 939883)

```
AttributeError: 'NoneType' object has no attribute 'split'

Stacktrace (most recent call last):

  File "django/core/handlers/base.py", line 111, in get_response
    response = callback(request, *callback_args, **callback_kwargs)
  File "newrelic/hooks/framework_django.py", line 485, in wrapper
    return wrapped(*args, **kwargs)
  File "amo/decorators.py", line 157, in wrapper
    return f(*args, **kw)
  File "amo/decorators.py", line 149, in wrapper
    return f(*args, **kw)
  File "waffle/decorators.py", line 36, in _wrapped_view
    return view(request, *args, **kwargs)
  File "addons/decorators.py", line 33, in wrapper
    return f(request, addon, *args, **kw)
  File "amo/decorators.py", line 32, in wrapper
    return func(request, *args, **kw)
  File "mkt/developers/decorators.py", line 49, in wrapper
    return fun()
  File "mkt/developers/decorators.py", line 28, in <lambda>
    *args, **kw)
  File "mkt/developers/views.py", line 320, in content_ratings_edit
    form.save(addon)
  File "mkt/developers/forms.py", line 1102, in save
    data = lib.iarc.utils.IARC_XML_Parser().parse_string(resp)
  File "lib/iarc/utils.py", line 133, in parse_string
    return self.parse(StringIO.StringIO(string))
  File "lib/iarc/utils.py", line 118, in parse
    data = self._process_iarc_items(data)
  File "lib/iarc/utils.py", line 74, in _process_iarc_items
    [s.strip() for s in v.split(',')])
```