Repetitive calls to `NSNull.Null.Handle`, e.g. from `NSArray.UnsafeGetItem<T>`,
means several (one by item), costly, calls into ObjC code - but it always
return the same (native) singleton.
This manually cache the managed `NSNull.Null` static value, once it's
initialized the first time, so future calls won't have the performance
penalty.
https://github.com/xamarin/xamarin-macios/issues/3544
The existing code was cheating and returned a managed only instance with a `nil` handle. That was fine in many cases but some API (e.g. UISegmentedControl) don't like that (i.e. don't react like a normal, empty NSArray was supplied). It's also the right thing to do since the current behaviour is not guaranteed to remain identical on future updates of the OS.
Unit tests updated.
Sebastien Pouliot
Rolf Bjarne Kvinge