Adding a NullAllowed to allow creating PSPDFViewControllers with empty d...

...ocuments.

I imagine there are some more [Export] methods that are called internally that pass nil arguments which cause the generated c# (without NullAllowed) to throw an error when it shouldn't.

Side note, I always keep thinking NullAllowed should be default in Xamarin.iOS with a NullNotAllowed as an optional parameter attribute.