Avoid boxing nullable values in BindingValue.

src/Avalonia.Base/Data/BindingValue.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
 using Avalonia.Utilities;
 
@@ -358,6 +359,7 @@ namespace Avalonia.Data
                 e);
         }
 
+        [Conditional("DEBUG")]
         private static void ValidateValue([AllowNull] T value)
         {
             if (value is UnsetValueType)