On upgrade, update grade items to 'attendance'

This causes a minor error after upgrade:

Notice: Invalid get_string() identifier: 'modulename' or component 'attforblock'. Perhaps you are missing $string['modulename'] = ''; in mod/attforblock/lang/en/attforblock.php? * line 6982 of /lib/moodlelib.php: call to debugging() * line 7682 of /lib/moodlelib.php: call to core_string_manager->get_string() * line 1201 of /grade/lib.php: call to get_string() * line 1249 of /grade/lib.php: call to grade_structure->get_element_icon() * line 319 of /grade/report/user/lib.php: call to grade_structure->get_element_header() * line 563 of /grade/report/user/lib.php: call to grade_report_user->fill_table_recursive() * line 305 of /grade/report/user/lib.php: call to grade_report_user->fill_table_recursive() * line 927 of /grade/report/user/lib.php: call to grade_report_user->fill_table() * line 129 of /course/user.php: call to grade_report_user_profilereport() in /var/www/htdocs/moodle/lib/weblib.php on line 2769
Invalid get_string() identifier: 'modulename' or component 'attforblock'. Perhaps you are missing $string['modulename'] = ''; in mod/attforblock/lang/en/attforblock.php?
line 6982 of /lib/moodlelib.php: call to debugging()
line 7682 of /lib/moodlelib.php: call to core_string_manager->get_string()
line 1201 of /grade/lib.php: call to get_string()
line 1249 of /grade/lib.php: call to grade_structure->get_element_icon()
line 319 of /grade/report/user/lib.php: call to grade_structure->get_element_header()
line 563 of /grade/report/user/lib.php: call to grade_report_user->fill_table_recursive()
line 305 of /grade/report/user/lib.php: call to grade_report_user->fill_table_recursive()
line 927 of /grade/report/user/lib.php: call to grade_report_user->fill_table()
line 129 of /course/user.php: call to grade_report_user_profilereport()

Requires attforblock grade column in the gradebook before upgrade. ie: mark a session, then upgrade.