Handle NoneType docstrings to support the -OO interpreter option

When running uWSGI with the `--optimize` option set to `2`, it passes the `-OO` option to the interpreter, which removes docstrings. The library fails to import in this environment with the following error due to attempted concatenation with an undefined variable. 

```
unsupported operand type(s) for +=: 'NoneType' and 'str'
```