lua - don't use nil to create empty caption

Author: cderv

src/resources/filters/quarto-pre/table-classes.lua

@@ -53,7 +53,7 @@ function table_classes()
       attr.classes = pandoc.List()
       tbl.caption.long[#tbl.caption.long] = pandoc.Plain(createTableCaption(caption_parsed, attr))
       if #quarto.utils.as_inlines(tbl.caption.long) == 0 then
-        tbl.caption.long = nil
+        tbl.caption.long = pandoc.Blocks({})
       end
       return tbl
     end,