MultiValueDict should use sentinel for default values

[Peter Larsen - Puzzel Solutions]

MultiValueDict has many methods that has a 'default' parameter, but sets the default to None. fx:
​https://github.com/django/django/blob/main/django/utils/datastructures.py#L129

This makes
mvdict.getlist('key', None)
return [], instead of the expected None.

[Simon Charette]

Assuming we want to change this behavior we'd have to go through a deprecation period as there might be code out there that expects [] to be returned from getlist(missing_key, None). Slightly related to #29356.

[Natalia Bidart]

I don't think we should change this. The behavior is exactly as ​documented for, QueryDict:

Returns a list of the data with the requested key. Returns an empty list if the key doesn’t exist and default is None. It’s guaranteed to return a list unless the default value provided isn’t a list.

It makes sense to me that getlist should always return a list. The intended usage is either to pass a non-None default or expect an empty list as the result. Changing the semantics of default would be a new feature, requiring not only a deprecation period but also community consensus.

(Note that MultiValueDict, on its own, is not a documented nor public data structure to reuse. It's the base structure for QueryDict and that's the behavior I'm analyzing and prioritizing with this triage.)