[FIX]mod-func-directory_last_sites.php: Fix undefined array key desc and maxdesc

This is to fix ERROR (E_WARNING): Undefined array key "desc" and ERROR (E_WARNING): Undefined array key "maxdesc" in mod-func-directory_last_sites.php on master when we are using this module in a wiki page like {module module="directory_last_sites"}.

If the key "desc" does not exist, set the variable $module_params['desc'] to the default value 'n'

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