Fix nullable event warning in interface events documentation (#47684)

* Initial plan

* Fix nullable event warning in interface events documentation

Co-authored-by: BillWagner <[email protected]>

---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: BillWagner <[email protected]>

Author: Copilot

docs/csharp/programming-guide/events/how-to-implement-interface-events.md

@@ -29,7 +29,7 @@ namespace ImplementInterfaceEvents
     }  
     public class Shape : IDrawingObject  
     {  
-        public event EventHandler ShapeChanged;  
+        public event EventHandler? ShapeChanged;  
         void ChangeShape()  
         {  
             // Do something here before the event…  

samples/snippets/csharp/VS_Snippets_VBCSharp/csProgGuideEvents/CS/Events.cs

@@ -686,21 +686,18 @@ public class MyEventArgs : EventArgs
     }
     public class Shape : IDrawingObject
     {
-        public event EventHandler ShapeChanged;
+        public event EventHandler? ShapeChanged;
         void ChangeShape()
         {
-            // Do something here before the event�
+            // Do something here before the event…
 
             OnShapeChanged(new MyEventArgs(/*arguments*/));
 
             // or do something here after the event.
         }
         protected virtual void OnShapeChanged(MyEventArgs e)
         {
-            if (ShapeChanged != null)
-            {
-                ShapeChanged(this, e);
-            }
+            ShapeChanged?.Invoke(this, e);
         }
     }
 }