Update Path.GetDirectoryName Snippet3 (#10853)

the current docs are outdated to NRT and confuse quotes
1. variable should be type string? not string
2. uses old-style format string (new would use interpolated syntax)
3. filepath reassignment should use conditional syntax
4. results suggest using single-quote [i.e. char not string], ugh
5. the final returned result is null but demo output shows '' which is factually visually correct but confusingly indistinuishable from "" emptystring

I propose replacement C# code for this Snippet3

Author: DickBaker

snippets/csharp/System.IO/Path/ChangeExtension/pathmembers.cs

@@ -61,31 +61,30 @@ public void Combine()
     public void GetDirectoryName()
     {
         // <Snippet3>
-        string filePath = @"C:\MyDir\MySubDir\myfile.ext";
-        string directoryName;
-        int i = 0;
-
-        while (filePath != null)
-        {
-            directoryName = Path.GetDirectoryName(filePath);
-            Console.WriteLine("GetDirectoryName('{0}') returns '{1}'",
-                filePath, directoryName);
-            filePath = directoryName;
-            if (i == 1)
-            {
-                filePath = directoryName + @"\";  // this will preserve the previous path
-            }
-            i++;
-        }
-        /*
-        This code produces the following output:
-
-        GetDirectoryName('C:\MyDir\MySubDir\myfile.ext') returns 'C:\MyDir\MySubDir'
-        GetDirectoryName('C:\MyDir\MySubDir') returns 'C:\MyDir'
-        GetDirectoryName('C:\MyDir\') returns 'C:\MyDir'
-        GetDirectoryName('C:\MyDir') returns 'C:\'
-        GetDirectoryName('C:\') returns ''
-        */
+    string? filePath = @"C:\MyDir\MySubDir\myfile.ext";
+    string? directoryName;
+    int i = 0;
+
+    while (filePath != null)
+    {
+        directoryName = Path.GetDirectoryName(filePath);
+        Console.WriteLine($"GetDirectoryName(\"{filePath}\") returns {directoryName ?? "NULL"}");
+
+        filePath = (i == 1)
+            ? directoryName + @"\"  // this will preserve the previous path
+            : directoryName;
+
+        i++;
+    }
+    /*
+    This code produces the following output:
+
+    GetDirectoryName("C:\MyDir\MySubDir\myfile.ext") returns C:\MyDir\MySubDir
+    GetDirectoryName("C:\MyDir\MySubDir") returns C:\MyDir
+    GetDirectoryName("C:\MyDir\") returns C:\MyDir
+    GetDirectoryName("C:\MyDir") returns C:\
+    GetDirectoryName("C:\") returns NULL
+    */
         // </Snippet3>
 
         Console.WriteLine();
@@ -378,4 +377,4 @@ public static void Main()
         pathSnippets.HasExtension();
         pathSnippets.IsPathRooted();
     }
-}
+}