Windows source build fails when CMake generator is unset

On Windows, `setup.py` can fail before invoking CMake when neither `CMAKE_GENERATOR` nor Ninja is present.

`get_cmake_generator()` then returns `None`, but the MSVC branch evaluates `x in cmake_generator`, which raises `TypeError: argument of type 'NoneType' is not a container or iterable`.

Leaving the generator unset is valid for CMake, which can select its default Visual Studio multi-config generator. The MSVC logic should therefore treat `None` as a multi-config generator instead of iterating over it.

Start in setup.py by reading get_cmake_generator() and the MSVC branch that checks the returned generator. Reproduce the Windows source-build path with neither CMAKE_GENERATOR nor Ninja configured, then verify that an unset generator no longer raises TypeError and CMake can select its default Visual Studio generator.

Written by the indexing model from the issue text.