parse-print roundtrip changes list formatting

[3b]

(3bmd::parse-string-and-print-to-stream "- a
- b
" t :format :markdown)

;; ->

"- a

- b
"

but those don't quite parse the same:

(3bmd::parse-doc "- a
- b
")
;; ((:BULLET-LIST (:LIST-ITEM (:PLAIN "a")) (:LIST-ITEM (:PLAIN "b"))))

(3bmd::parse-doc "- a

- b
")
;; ((:BULLET-LIST (:LIST-ITEM (:PARAGRAPH "a")) (:LIST-ITEM (:PARAGRAPH "b"))))

[3b]

(it also changes the bullet character, for example from * to - but that doesn't affect how the result is parsed, so probably not worth fixing)