Add nullability marks for better Swift compatibility

It's better to mark properties and parameters nullability in your Objective-C SDK. Please, look at [the article](https://developer.apple.com/documentation/swift/objective-c_and_c_code_customization/designating_nullability_in_objective-c_apis), or at the [other one](https://developer.apple.com/swift/blog/?id=25) where it's explained.

A quick example — I don't know am I allowed to pass `nil` as `value` of `AMPIdentify().set("key", value: isNilValueAllowedHere)`