<filesystem>: create_directories() should throw for empty paths

[StephanTLavavej]

Reported by Ville Voutilainen:

C:\Temp>type meow.cpp

#include <filesystem>
#include <iostream>
using namespace std;
using namespace std::filesystem;

int main() {
    cout << boolalpha;

    const path p{};

    try {
        const bool result = create_directories(p);

        cout << "create_directories returned " << result << ".\n";
    } catch (const filesystem_error& e) {
        cout << "create_directories threw " << e.what() << ".\n";
    }
}

C:\Temp>cl /EHsc /nologo /W4 /std:c++17 /MTd meow.cpp && meow
meow.cpp
create_directories returned false.

This behavior differs from libstdc++ on Compiler Explorer:

create_directories threw filesystem error: cannot create directories: Invalid argument [].

And from libc++ on Compiler Explorer:

create_directories threw filesystem error: in create_directory: No such file or directory [""].