Non-dict default with a dict value breaks `Group.all`

[Flame442]

What Red version are you using?

3.5.20

What were you trying to do?

If a config key defaults to a non-dict value (ie, that key is not a Group), storing a dict to that key causes scope.all() to error when trying to recursively fetch the default values for that dict's keys. I am only able to reproduce this issue for .all, directly accessing the key does not produce an error.

It is unclear whether this should be considered a user error (mixing Groups and Values) and should be entirely prevented or if this should be considered a bug with Config and an additional check should be added in nested_update to handle this edge case.

What did you expect to happen?

No error

What actually happened?

Traceback (most recent call last):
  File "<eval command - snippet #4>", line 4, in func
    return await c.all()
           ^^^^^^^^^^^^^
  File "\redbot\core\config.py", line 307, in _get
    return self.nested_update(raw, default)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "\redbot\core\config.py", line 553, in nested_update
    result = self.nested_update(value, defaults.get(key, {}))
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "\redbot\core\config.py", line 556, in nested_update
    defaults[key] = pickle.loads(pickle.dumps(current[key], -1))
    ~~~~~~~~^^^^^
TypeError: 'NoneType' object does not support item assignment

How can we reproduce this error?

Example of broken code

config.register_global(testvar=None)
await config.testvar.set({"a": 4})
return await config.all() # Errors

Example alternative that does not break (not mixing groups and values)

config.register_global(testvar={})
await config.testvar.set({"a": 4})
return await config.all() # Works

This issue applies to values at all depths

cconfig.register_global(testvar={"b": None})
await config.testvar.set({"a": 4, "b": {"c": 56}})
return await config.all() # Errors

Anything else?

Found by unknown

Discussion for reference:
https://discord.com/channels/133049272517001216/160386989819035648/1467179303262486613
https://discord.com/channels/133049272517001216/171665724262055936/1467617566204891339