<nullable> cause MvxWpfView error
## 🐛 Bug Report

**Error	CS0263	Partial declarations of 'SomeView' must not specify different base classes**

### Expected behavior
the nullable attribute shouldn't affect the `MvxWpfView`

### Reproduction steps
create a mvvmcross wpf project and add the `nullable` attribut in project settings
```
<Project  Sdk="Microsoft.NET.Sdk.WindowsDesktop>
 ....
<PropertyGroup>  
<Nullable>enable</Nullable>  
</PropertyGroup>  
<Project>  
```
create a view:
```
<views:MvxWpfView  
x:Class="BlaBla.SomeView"  
xmlns:views="clr-namespace:MvvmCross.Platforms.Wpf.Views;assembly=MvvmCross.Platforms.Wpf"  
x:TypeArguments="SomeViewModel">  
<Grid>  
...  
</Grid>  
</views:MvxWpfView>  
```
with a basic code behind:
```
 public partial class SomeView : MvxWpfView<SomeViewModel>
    {
        public SomeView ()
        {
            InitializeComponent();
        }
    }
```
### Configuration

**Version:** 8.0.1

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