fix: return empty element as suggested

Author: mcanouil

src/resources/filters/quarto-pre/shortcodes-handlers.lua

@@ -310,7 +310,7 @@ function handlePagebreak()
   if FORMAT == 'docx' then
     return pandoc.RawBlock('openxml', pagebreak.ooxml)
   elseif FORMAT == 'pptx' then
-    return pandoc.Para{pandoc.Str ''}
+    return {}
   elseif FORMAT:match 'latex' then
     return pandoc.RawBlock('tex', pagebreak.latex)
   elseif FORMAT:match 'odt' then