Support None responses in results

Avoids these annoying messages printed on stdout:
```
Defaulting no known way to handle <class 'NoneType'>: None
Defaulting no known way to handle <class 'NoneType'>: None
Defaulting no known way to handle <class 'NoneType'>: None
Defaulting no known way to handle <class 'NoneType'>: None
```

**Edit**: also made the print side effect a proper warning.