Never convert null to UnsetValue in bindings

Revert ToggleButton.IsChecked hack

src/Avalonia.Base/Data/Converters/DefaultValueConverter.cs

@@ -30,7 +30,7 @@ namespace Avalonia.Data.Converters
         {
             if (value == null)
             {
-                return targetType.IsValueType ? AvaloniaProperty.UnsetValue : null;
+                return null;
             }
 
             if (typeof(ICommand).IsAssignableFrom(targetType) && value is Delegate d && d.Method.GetParameters().Length <= 1)

src/Avalonia.Controls/Primitives/ToggleButton.cs

@@ -20,7 +20,7 @@ namespace Avalonia.Controls.Primitives
                 nameof(IsChecked),
                 o => o.IsChecked,
                 (o, v) => o.IsChecked = v,
-                unsetValue: null,
+                unsetValue: false,
                 defaultBindingMode: BindingMode.TwoWay);
 
         /// <summary>

tests/Avalonia.Base.UnitTests/Data/Core/BindingExpressionTests.cs

@@ -78,18 +78,6 @@ namespace Avalonia.Base.UnitTests.Data.Core
             GC.KeepAlive(data);
         }
 
-        [Fact]
-        public async Task Should_Coerce_Get_Null_Double_String_To_UnsetValue()
-        {
-            var data = new Class1 { StringValue = null };
-            var target = new BindingExpression(ExpressionObserver.Create(data, o => o.StringValue), typeof(double));
-            var result = await target.Take(1);
-
-            Assert.Equal(AvaloniaProperty.UnsetValue, result);
-
-            GC.KeepAlive(data);
-        }
-
         [Fact]
         public void Should_Convert_Set_String_To_Double()
         {
@@ -249,19 +237,6 @@ namespace Avalonia.Base.UnitTests.Data.Core
             GC.KeepAlive(data);
         }
 
-        [Fact]
-        public void Should_Coerce_Setting_Null_Double_To_Default_Value()
-        {
-            var data = new Class1 { DoubleValue = 5.6 };
-            var target = new BindingExpression(ExpressionObserver.Create(data, o => o.DoubleValue), typeof(string));
-
-            target.OnNext(null);
-
-            Assert.Equal(0, data.DoubleValue);
-
-            GC.KeepAlive(data);
-        }
-
         [Fact]
         public void Should_Coerce_Setting_UnsetValue_Double_To_Default_Value()
         {