avoid breakQuartoMd horizontal rule bug

i think this is a quarto bug and i've submitted a PR to fix it
but breakQuartoMd carries a lot of weight so we have to be careful

pandoc accepts a horizontal rule with only a blank line after
so this was rendering ok but it was losing a bunch of marimo cells
and the document failed to run

Author: gordonwoodhull

index.qmd

@@ -5,6 +5,7 @@ engine: marimo
 ---
 
 # 🟢 🔵 🏝️
+
 ---