fold_sum_1, fold_mul_1 need init value for null parameter case about articles

[wichtounet]

Hi Richard,

I am almost sure this would work with GCC 9.2 as long as you use -std=c++17.
As for MSVC, I would indeed not be surprised if this didn't work.

Since this example is supposed to show the two possibilities of the C++17 standard (with and without init parameter), I am not going to update it. This is perfectly fine according to the standard.
I am not using MSVC and I am almost sure a lot of this code in this repository would not compile with it.

Thanks for letting me know!