Fix crashes when using std::locale{""} with an invalid LC_ALL

[JoshuaVandaele]

Tested on Windows and Linux. I've also slipped in a semi-related clean up in PostProcessing.cpp that isn't worth its own PR imo but if anyone feels strongly about it I can remove it.

[JosJuice]

Do you foresee any uses of this where std::nullopt would lead to something other than falling back to std::locale::classic? If not, it could be better to return std::locale::classic from the function instead of std::nullopt.

[JoshuaVandaele]

Do you foresee any uses of this where you're going to do something other than falling back to std::locale::classic? If not, it could be better to return std::locale::classic from the function instead of std::nullopt.

I can't come up with one, so let's do that instead