Calling ClearValue if a color is null on WinUI is incorrect behavior

[PureWeen]

Description

maui/src/Core/src/Platform/Windows/ViewExtensions.cs

Line 155 in 8966710

nativeControl.ClearValue(property);

If a property is set via a TemplateBinding or a ThemeResource inside a ControlTemplate then calling ClearValue will just reset the property to the DP default

If the color is null then UpdateProperty should just do nothing

Steps to Reproduce

Call UpdateProperty on a Control inside ControlTemplate with a null color and you'll see it doesn't actually revert to a relevant default value

For example, when setting the TabBar we are setting the background on the following control that's set via a ThemeResource

https://github.com/microsoft/microsoft-ui-xaml/blob/991c831e4ac5b58591621f904c2f94f5a9a5bc4f/dev/NavigationView/NavigationView.xaml#L281

If the user sets the color back to null then clearing that DP doesn't re-instate the ThemeResource

Version with bug

Preview 13

Last version that worked well

Unknown/Other

Affected platforms

Windows

Affected platform versions

WinUI

Did you find any workaround?

No response

Relevant log output

No response