An empty universe would not have any information, or state change, and wouldn’t trigger any computation, therefore, it wouldn’t exist.
But if I include it as a variant in my Universe enum, then doesn't it exist anyway, at least as a concept, even if it's never constructed?
Then it's not a real universe but a mere symbol, I'd say. "Map vs. territory"
And in order for that symbol to exist (be expressed / thought of / conceived), it must be a part of some other existing universe that allows that...
But in which universe does that "map" exist? Boom, you've just created an universe containing a map out of a never-constructed-universe-containing-nothing.