fix(export.markdown): fix error on unexported verbatim tags without p…

…arameters (nvim-neorg#1280)

fixes nvim-neorg#1277

lua/neorg/modules/core/export/markdown/module.lua

@@ -612,7 +612,7 @@ module.public = {
             end,
 
             ["ranged_verbatim_tag"] = function(output)
-                if output[2]:match("^[ \t]+$") then
+                if output[2] and output[2]:match("^[ \t]+$") then
                     table.remove(output, 2)
                 end