It prevents this bug:
```python
viewcode can't import None, failed with error "__import__() argument 1 must be str, not None"
Traceback (most recent call last):
File "/usr/local/lib/python3.5/dist-packages/sphinx/ext/viewcode.py", line 28, in _get_full_modname
return get_full_modname(modname, attribute)
File "/usr/local/lib/python3.5/dist-packages/sphinx/util/__init__.py", line 263, in get_full_modname
__import__(modname)
TypeError: __import__() argument 1 must be str, not None
```