Nullable Enable
## 🏗 Enhancement Proposal

The way you handle nullable objects has changed in C#8. Now by default an object is not nullable. But since this change is very difficult. It needs to be enabled with

`<Nullable>enable</Nullable>`

To be able to make this little change. You can enable nullable in a single file with.

`#nullable enable`

### Pitch

<!--- How will this refactor improve contributors' lives? Input as many details as possible! --->

### Platforms affected (mark all that apply)
- [x] :iphone: iOS
- [x] :robot: Android
- [x]  :checkered_flag: WPF
- [x] :earth_americas: UWP
- [x] :apple: MacOS
- [x] :tv: tvOS
- [x] :monkey: Xamarin.Forms
