You could write the clojure version in the exact same way (well, you wouldn't say null?, but, basically the same) as the lisp one. But it wouldn't be as efficient given the various ways sequences might be implemented in Clojure. That makes sense as part of the standard library, IMO---users of keep don't need to care about whether their sequence is chunked or whatever. And it makes sense that there should be a variety of implementers of sequences: there's more to life than singly-linked lists.

(Even Guy Steele, who should know, recommended a couple years ago that language designers omit cons from their langauges!)