The check for null in 'Show' method breaks WPF navigation
## 🔙 Regression

It is necessary to improve the 'CreatePresentationAttribute' method of the 'MvxWpfViewPresenter' class so it fills in the fields (ViewModelType & ViewType) of the attribute being created, because a null check has been added to the 'Show' method  of the 'MvxAttributeViewPresenter' class. See screenshots below.

### Old (and correct) behavior

The application is launched and the 'MvxWpfView' is displayed to the user (inside of 'MvxWindow').

### Current behavior

The application is launched and the user only sees root 'MvxWindow' alone.

### Reproduction steps

Update your WPF application from MvvmCross 7 to MvvmCross 8 then run it. The WPF application should use 'MvxWpfView' for your startup ViewModel.

### Configuration

**Version:** MvvmCross 8..0.2

**Platform:** 
- [ ] :iphone: iOS
- [ ] :robot: Android
- [x] :checkered_flag: WPF
- [ ] :earth_americas: UWP
- [ ] :apple: MacOS
- [ ] :tv: tvOS
- [ ] :monkey: Xamarin.Forms

### Suggestions to fix the problem

Change the code in 88 line of 'MvxWpfViewPresenter' class from:
`return new MvxContentPresentationAttribute();`
to this code:
`
return new MvxContentPresentationAttribute()
{
    ViewModelType = viewModelType,
    ViewType = viewType
};
`

in the 'MvxWpfViewPresenter' class:
![default-attribute](https://user-images.githubusercontent.com/6156565/137634610-0446fbb3-9b4c-492a-bce2-56268d6829dd.png)

in the 'MvxAttributeViewPresenter' class:
![always-null-for-WPF-attribute](https://user-images.githubusercontent.com/6156565/137634663-ab37d7ab-c5d3-4b40-94d8-efd7456d93e0.png)
