stubgen incorrectly represents function arguments whose default is None as having no default #18757

To Reproduce

I've create a repo that reproduces this issue. It uses this simple code:

class Cls:
    def __init__(self, a = 1, b = None):
        self.a = a
        self.b = b

Note how b has a default value, but that value is None. This is different from an argument that has no default.

To reproduce:

git clone https://github.com/multimeric/StubgenBug
cd StubgenBug/
uv run stubgen --inspect-mode -m tmp_dmnkkm8uuq
cat out/tmp_dmnkkm8uuq/__init__.pyi

Expected Behavior
I would expect b to indicate the default value exists:

class Cls:
    def __init__(self, a: int = ..., b = ...) -> None: ...

Actual Behavior
b doesn't have a default:

class Cls:
    def __init__(self, a: int = ..., b) -> None: ...

Your Environment

Python 3.12.6
stubgen 1.15.0