fix: Win32 nullable

src/Windows/Avalonia.Win32/Automation/RootAutomationNode.cs

@@ -42,7 +42,7 @@ namespace Avalonia.Win32.Automation
             return GetOrCreate(focus);
         }
 
-        public void FocusChanged(object sender, EventArgs e)
+        public void FocusChanged(object? sender, EventArgs e)
         {
             RaiseFocusChanged(GetOrCreate(Peer.GetFocus()));
         }

src/Windows/Avalonia.Win32/Interop/Automation/ISelectionItemProvider.cs

@@ -1,3 +1,4 @@
+#nullable enable
 using System;
 using System.Runtime.InteropServices;