Ensure that Nulls are ignored in creating slide shows.

Also ensure that Nulls are ignored in sectionification
by `makeSections`.

Closes #8155.

src/Text/Pandoc/Shared.hs

@@ -589,6 +589,7 @@ makeSections numbering mbBaseLevel bs =
     xs' <- go xs
     rest' <- go rest
     return $ Div attr xs' : rest'
+  go (Null:xs) = go xs
   go (x:xs) = (x :) <$> go xs
   go [] = return []
 

src/Text/Pandoc/Slides.hs

@@ -30,7 +30,7 @@ getSlideLevel = go 6
 
 -- | Prepare a block list to be passed to makeSections.
 prepSlides :: Int -> [Block] -> [Block]
-prepSlides slideLevel = ensureStartWithH . splitHrule . extractRefsHeader
+prepSlides slideLevel = ensureStartWithH . splitHrule . extractRefsHeader . filter (/= Null)
   where splitHrule (HorizontalRule : Header n attr xs : ys)
                        | n == slideLevel = Header slideLevel attr xs : splitHrule ys
         splitHrule (HorizontalRule : xs) = Header slideLevel nullAttr [Str "\0"] :

test/command/8155.md

@@ -0,0 +1,19 @@
+```
+% pandoc -f native -t revealjs
+[ Null
+, Header
+    2
+    ( "header-after-null" , [] , [] )
+    [ Str "Header"
+    , Space
+    , Str "after"
+    , Space
+    , Str "null"
+    ]
+]
+^D
+<section id="header-after-null" class="title-slide slide level2">
+<h2>Header after null</h2>
+
+</section>
+```