Fix Click in SplitButton to be nullable

src/Avalonia.Controls/SplitButton/SplitButton.cs

@@ -24,7 +24,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Raised when the user presses the primary part of the <see cref="SplitButton"/>.
         /// </summary>
-        public event EventHandler<RoutedEventArgs> Click
+        public event EventHandler<RoutedEventArgs>? Click
         {
             add => AddHandler(ClickEvent, value);
             remove => RemoveHandler(ClickEvent, value);
@@ -34,7 +34,9 @@ namespace Avalonia.Controls
         /// Defines the <see cref="Click"/> event.
         /// </summary>
         public static readonly RoutedEvent<RoutedEventArgs> ClickEvent =
-            RoutedEvent.Register<SplitButton, RoutedEventArgs>(nameof(Click), RoutingStrategies.Bubble);
+            RoutedEvent.Register<SplitButton, RoutedEventArgs>(
+                nameof(Click),
+                RoutingStrategies.Bubble);
 
         /// <summary>
         /// Defines the <see cref="Command"/> property.