[Win][AncestralBinding] throws COMException

[Xiaoy312]

Current behavior

<ListView ItemsSource="{Binding Files, Mode=OneWay}">
	<ListView.ItemTemplate>
		<DataTemplate>
			<Button IsEnabled="{utu:AncestorBinding AncestorType=ListView, Path=DataContext.IsBusy}" />

throws COMException

System.Runtime.InteropServices.COMException (0x80004005)
   at WinRT.ExceptionHelpers.<ThrowExceptionForHR>g__Throw|39_0(Int32 hr)
   at WinRT.ExceptionHelpers.ThrowExceptionForHR(Int32 hr)
   at ABI.Microsoft.UI.Xaml.IFrameworkElementOverridesMethods.MeasureOverride(IObjectReference _obj, Size availableSize)
   at Microsoft.UI.Xaml.FrameworkElement.MeasureOverride(Size availableSize)
   at Microsoft.UI.Xaml.FrameworkElement.Microsoft.UI.Xaml.IFrameworkElementOverrides.MeasureOverride(Size availableSize)
   at ABI.Microsoft.UI.Xaml.IFrameworkElementOverrides.Do_Abi_MeasureOverride_0(IntPtr thisPtr, Size availableSize, Size* result)

---OR---

<Page ...>
	<Button IsEnabled="{utu:AncestorBinding AncestorType=Page, Path=DataContext.IsBusy}" />

throws XamlParseException

Microsoft.UI.Xaml.Markup.XamlParseException: 'The text associated with this error code could not be found.

Failed to assign to property 'Microsoft.UI.Xaml.Controls.Control.IsEnabled' because the type 'Windows.Foundation.Object' cannot be assigned to the type 'Windows.Foundation.Boolean'. [Line: 14 Position: 11]'

throws exception.

Expected behavior

^ should not throw any exception.

How to reproduce it (as minimally and precisely as possible)

Nuget Package: Uno.Toolkit.WinUI@6.0.18
Affected platform(s): Windows

[Xiaoy312]

In AncestorBinding::ProvideValue, we are first returning a default null value when the binding isnt ready to happen. For some dependency property, it would be illegal.
Given that we must return something by interface's contract, we should probably return the default value for the target property, or better yet the current value on this property.