fix: DataFrame.assign None

[cmp0xff]

- [ ] Closes #xxxx (Replace xxxx with the Github issue number)

• Tests added: Please use assert_type() to assert the type of any return value

Describe the bug

Assigning a None column using DataFrame.assign causes a typing error.

To Reproduce

Provide a minimal runnable pandas example that is not properly checked by the stubs.

import pandas as pd

pd.DataFrame({"a": [1]}).assign(c=None)

Indicate which type checker you are using (mypy or pyright).

mypy

Show the error message received from that type checker while checking your example.

assign.py:3:35: error: Argument "c" to "assign" of "DataFrame" has incompatible type "None"; expected "ExtensionArray | ndarray[Any, Any] | Index[Any] | Series[Any] | str | bytes | date | datetime | timedelta | <7 more items> | complex | integer[Any] | floating[Any] | complexfloating[Any, Any] | Callable[[DataFrame], ExtensionArray | ndarray[Any, Any] | Index[Any] | Series[Any] | str | bytes | date | datetime | timedelta | <7 more items> | complex | integer[Any] | floating[Any] | complexfloating[Any, Any]]"  [arg-type]
Found 1 error in 1 file (checked 1 source file)

Please complete the following information

OS

Windows

OS Version

10

python version

3.13.2

version of type checker

1.16.0

version of installed pandas-stubs

2.2.3.250527

[Dr-Irv]

Thanks @cmp0xff