Correctly handle UnsetValue/DoNothing in setters.

Just need to call `BindingValue<T>.FromUntyped` instead of doing a conditional, because otherwise we run the risk of passing `UnsetValue` to the `BindingValue<T>` ctor which will throw in that case.

src/Avalonia.Styling/Styling/PropertySetterBindingInstance.cs

@@ -164,7 +164,7 @@ namespace Avalonia.Styling
 
         private void ConvertAndPublishNext(object? value)
         {
-            _value = value is T v ? v : BindingValue<T>.FromUntyped(value);
+            _value = BindingValue<T>.FromUntyped(value);
 
             if (_isActive)
             {