From the source it expects that the second parameter (conversion), can
be None:
```python
def convert_field(self, value, conversion):
# do any conversion on the resulting object
if conversion is None:
return value
...
```
Since at least 17 years according to git blame, as None is refered as
early as 11529195cae2438a3ac003babcb1b11af67c4037
And still present in main branch:
https://github.com/python/cpython/blob/ef3ceab09d2d0959c343c662461123d5b0e0b64b/Lib/string.py
