Hidden viewer eval results produce vertical whitespace

[zampino]

Some code like

{::clerk/visibility {:code :hide :result :hide}}
;; ---
(clerk/eval-cljs 1)
(clerk/eval-cljs 2)
(clerk/eval-cljs 3)
;; ---
1
2
3
;; ---

renders as

[zampino]

A regression from #575.