StringEx.Join skips nulls

[NN---]

The code has invalid check for null: https://github.com/theraot/Theraot/blob/master/Framework.Core/System/StringEx.join.cs#L22

https://dotnetfiddle.net/Widget/fZrbN2

using System;

public class Program
{
    public static void Main()
    {
        Console.WriteLine(string.Join(",", new[]{"a","b",null}));
    }
}

a,b,

[NN---]

Fixed.