Fix NullReferenceException in ReflectionExtensions.GetProperties()

In iOS, due to the way the Linker works, some PropertyInfo.GetMethod()
calls might return null, so check before use value.

MvxSetup will use reflection to get all types and then search for Views and ViewModels. Problem is when we have a class that inherits from something like `UIView`; the setup will fail due to the Linker remove some properties from compilation (thus PropertyInfo.GetMethod returns null).

This fix is to just exclude any null from the list.