Shared: improve isTight.

If a list has an empty item, this should not count against
its being a tight list.

Closes #5857.

src/Text/Pandoc/Shared.hs

@@ -623,7 +623,7 @@ onlySimpleTableCells = all isSimpleCell . concat
 
 -- | Detect if a list is tight.
 isTightList :: [[Block]] -> Bool
-isTightList = all firstIsPlain
+isTightList = all (\item -> firstIsPlain item || null item)
   where firstIsPlain (Plain _ : _) = True
         firstIsPlain _             = False
 

test/command/5857.md

@@ -0,0 +1,11 @@
+```
+% pandoc -f native -t markdown
+[BulletList
+ [[Plain [Str "a"]]
+ ,[]
+ ,[Plain [Str "b"]]]]
+^D
+-   a
+-   
+-   b
+```