Fix None check in variant caster

[yosh-matsuda]

This PR fixes the missing None check in the std::variant caster.

The following code produces SIGABRT at runtime since the change in 1220156;

m.def("variant_copyable_none", [](std::variant<int, Copyable, std::monostate> &) {}, nb::arg("x").none());

t.variant_copyable_none(None)

This PR include

• Improve test coverage
• Revert the None check
• Refactoring of redundant code

[wjakob]

Thank you!

[wjakob]

This was kind of a serious issue that also needed to be fixed elsewhere. See commit 5f25ae0.