Default presentation doesn't fall back to MvxContentPresentation
## 🔙 Regression

<!--- Summary description of the regression --->

As described in #4201 there is a regression in how the presenter behaves when no presentation attribute is added on a view.

### Old (and correct) behavior

Previously when not specifying any presentation attribute on views the presenter would assume Content Presentation. 

### Current behavior

Now you have to explicitly use `[MvxContentPresentation]` on your WPF view or any other presentation attribute.

### Reproduction steps

1. Open the [TipCalc sample](https://github.com/MvvmCross/MvvmCross-Samples/tree/master/TipCalc)
2. Remove this line https://github.com/MvvmCross/MvvmCross-Samples/blob/master/TipCalc/TipCalc.WPF/Views/TipView.xaml.cs#L8
3. Nothing shows up in the main window

### Configuration

**Version:** 6.x

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