Actually I don't see any nulls:
given:
static void Main()
{
string[] testArray = new string[]
{
"aa",
"ab",
"ac",
"ad",
"ab",
"af"
};
Array.Sort(testArray, StringComparer.InvariantCulture);
Array.ForEach(testArray, x => Console.WriteLine(x));
}
I obtained: