fix: pagebreak breaks PowerPoint

Fixes #11893

Author: mcanouil

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

@@ -309,6 +309,8 @@ function handlePagebreak()
 
   if FORMAT == 'docx' then
     return pandoc.RawBlock('openxml', pagebreak.ooxml)
+  elseif FORMAT == 'pptx' then
+    return pandoc.Para{pandoc.Str ''}
   elseif FORMAT:match 'latex' then
     return pandoc.RawBlock('tex', pagebreak.latex)
   elseif FORMAT:match 'odt' then
@@ -326,4 +328,4 @@ function handlePagebreak()
     return pandoc.Para{pandoc.Str '\f'}
   end
 
-end
+end