fix: handle empty :Org arguments. (#952)

Author: DoloresHA

lua/orgmode/org/global.lua

@@ -122,7 +122,7 @@ end
 
 vim.api.nvim_create_user_command('Org', function(opts)
   local item = resolve_item(opts.fargs)
-  if item and (type(item) == 'function' or getmetatable(item).__call) then
+  if item and (type(item) == 'function' or (getmetatable(item) and getmetatable(item).__call)) then
     return item()
   end
   require('orgmode.utils').echo_error(('Invalid command "Org %s"'):format(opts.args))