iOS FlutterViewController splashScreenView made nullable

Summary

Context

Description of change

Migration guide

Timeline

References

Summary

The FlutterViewController property splashScreenView has been changed from nonnull to nullable.

Old declaration of splashScreenView:

New declaration of splashScreenView:

Context

Prior to this change, on iOS the splashScreenView property returned nil when no splash screen view
was set, and setting the property to nil removed the splash screen view. However, the 
splashScreenView API was incorrectly marked nonnull. This property is most often used
when transitioning to Flutter views in iOS add-to-app scenarios.

Description of change

While it was possible in Objective-C to work around the incorrect nonnull annotation by setting
splashScreenView to a nil UIView, in Swift this caused a compilation error: