Fix Array.Sort in the presence of nulls (#41234)

* Add failing test case

* Fix Array.Sort in the presence of nulls

Author: svick

src/libraries/System.Private.CoreLib/src/System/Collections/Generic/ArraySortHelper.cs

@@ -474,7 +474,7 @@ private static int PickPivotAndPartition(Span<T> keys)
                 if (pivot == null)
                 {
                     while (Unsafe.IsAddressLessThan(ref leftRef, ref nextToLastRef) && (leftRef = ref Unsafe.Add(ref leftRef, 1)) == null) ;
-                    while (Unsafe.IsAddressGreaterThan(ref rightRef, ref zeroRef) && (rightRef = ref Unsafe.Add(ref rightRef, -1)) == null) ;
+                    while (Unsafe.IsAddressGreaterThan(ref rightRef, ref zeroRef) && (rightRef = ref Unsafe.Add(ref rightRef, -1)) != null) ;
                 }
                 else
                 {

src/libraries/System.Runtime/tests/System/ArrayTests.cs

@@ -3326,6 +3326,7 @@ public static IEnumerable<object[]> Sort_SZArray_TestData()
             yield return new object[] { new string[] { "5", "2", "9", "8", "4", "3", "2", "4", "6" }, 0, 9, new StringComparer(), new string[] { "2", "2", "3", "4", "4", "5", "6", "8", "9" } };
             yield return new object[] { new string[] { "5", null, "2", "9", "8", "4", "3", "2", "4", "6" }, 0, 10, new StringComparer(), new string[] { null, "2", "2", "3", "4", "4", "5", "6", "8", "9" } };
             yield return new object[] { new string[] { "5", null, "2", "9", "8", "4", "3", "2", "4", "6" }, 3, 4, new StringComparer(), new string[] { "5", null, "2", "3", "4", "8", "9", "2", "4", "6" } };
+            yield return new object[] { new string[] { null, null, null, null, null, "foo", null, null, null, null, null, "bar", null, null, null, null, null }, 0, 17, null, new string[] { null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, "bar", "foo" } };
             yield return new object[] { new int[] { 1, 2, 3, 4 }, 0, 4, null, new int[] { 1, 2, 3, 4 } };
             yield return new object[] { new int[] { 4, 3, 2, 1 }, 0, 4, null, new int[] { 1, 2, 3, 4 } };
             yield return new object[] { new int[] { 4, 3, 2, 1 }, 1, 2, null, new int[] { 4, 2, 3, 1 } };