Returning UnsetValue in IValueConverter.ConvertBack is reported as a binding error

[MrJul]

Describe the bug

Returning UnsetValue in IValueConverter.ConvertBack is reported as a binding error after the binding system refactor (#13970) in 11.1.0-rc2.

System.InvalidCastException: Could not convert '(unset)' (Avalonia.UnsetValueType) to System.Object.

In 11.0, no errors happen.

To Reproduce

View:

<Window
  xmlns="https://github.com/avaloniaui"
  xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'
  xmlns:local="clr-namespace:Sandbox"
  x:Class="Sandbox.MainWindow"
  x:DataType="local:MainViewModel">

  <Window.Resources>
    <local:SomeConverter x:Key="SomeConverter" />
  </Window.Resources>

  <Window.DataContext>
    <local:MainViewModel />
  </Window.DataContext>

  <TextBox Text="{Binding Value, Converter={StaticResource SomeConverter}}" />

</Window>

VM:

#nullable enable

namespace Sandbox;

public class MainViewModel
{
    public object? Value { get; set; }
}

Converter:

#nullable enable

using System;
using System.Globalization;
using Avalonia;
using Avalonia.Data.Converters;

namespace Sandbox;

public sealed class SomeConverter : IValueConverter
{
    public object? Convert(object? value, Type targetType, object? parameter, CultureInfo culture)
        => value;

    public object ConvertBack(object? value, Type targetType, object? parameter, CultureInfo culture)
        => AvaloniaProperty.UnsetValue;
}

Type anything in the textbox.

Expected behavior

No errors are reported.

Avalonia version

11.1.0-rc2, master 376a841

OS

No response

Additional context

No response