Yes, you could argue that you will only use null for an actually empty result. But two things: 1) You won't keep to it because the compiler won't force you to do so, and 2) potential contributors will not comply. When using optionals/either/enums, compiler will not allow a wildcard result (such as null).
.empty is just more semantically specific, and if you read a method's API for the first time, semantics helps you grasp it a lot faster. That's the reason I'm using enums in Swift all over the place now. Compared to the nilling out of Objective-C, I can be way more confident that method results really are what they claim they are.