autodoc_preserve_defaults=True does not work for mixture of keyword only arguments with/without defaults

[saggitar]

Describe the bug

If I understand PEP 0570 correctly, the following is a valid signature of a class method:

class Thing:
    def __init__(
            self, 
            kw_or_pos_without_default, 
            kw_or_pos_with_default=None, 
            *,
            kw_without_default,
            kw_with_default="Foo"
    ):
        pass

When documenting this with autodoc and autodoc_preserve_defaults=True, sphinx.ext.autodoc.preserve_defaults.update_defvalue generates a DefaultValue with name=None for the kw_with_default arguments. This later raises an exception in sphinx.util.inspect.object_description since the DefaultValue.__repr__ dunder method now returns None instead of a string.

Basically what happens is that ast generates a None value in the kw_defaults of the arguments since the first keyword argument is required, but update_defvalue simply ignores that argument because the default is empty. This leaves the None in the kw_defaults to be picked up when the keyword argument with default value is processed -- instead of the actual default.
This can't be resolved by the unparse call which therefore simply returns None, which ends up as the name of the DefaultValue.

Imo this could simply be resolved by poping the corresponding None from the kw_defaults if a KW_ONLY parameter with empty default is encountered.

How to Reproduce

Create a module with contents

class Thing:
    def __init__(
            self, 
            kw_or_pos_without_default, 
            kw_or_pos_with_default=None, 
            *,
            kw_without_default,
            kw_with_default="Foo"
    ):
        pass

and auto-document while setting autodoc_preserve_defaults=True in your conf.py

Make sure sphinx tries to document all parameters, (since it's a __init__ method, they will be documented when the autodoc directive has :undoc-members:, if you try the same with a module level method you need to document the parameters)

test.zip

Expected behavior

The correct default value should be documented. The Warning Message also is pretty worthless (probably the value should not be
formatted with a simple %s but instead with a %r?)

Your project

https://github.com/sphinx-doc/sphinx/files/8253301/test.zip

OS

Any

Python version

Tested with versions > 3.8

Sphinx version

4.4.0

Sphinx extensions

sphinx.ext.autodoc

[tk0miya]

Thank you for a good report! As you said, we have to consume None item from kw_defaults. I think #10321 will resolve this problem.