Generate code with [AllowNullAttribute] on nullable properties

Accessing the `View` property of a view controller for the first time causes it to be loaded. It can later be assigned `null` to free memory.

At the moment the property is declared as follows:

```csharp
public virtual UIView? View
```

I believe it should be declared as:

(corrected)

```csharp
[AllowNull]
public virtual UIView View
```

I don't really know enough about how the bindings are generated to know if that is possible.