When attempting to delete a group in Course Administration->Users->Groups there was a fatal error (group was not empty):

Coding error detected, it must be fixed by a programmer: moodle_database::get_in_or_equal() does not accept empty arrays

More information about this error

Debug info:
Error code: codingerror
Stack trace:
line 646 of /lib/dml/moodle_database.php: coding_exception thrown
line 802 of /mod/quiz/locallib.php: call to moodle_database->get_in_or_equal()
line 1628 of /mod/quiz/locallib.php: call to quiz_update_open_attempts()
line ? of unknownfile: call to quiz_groups_group_deleted_handler()
line 299 of /lib/eventslib.php: call to call_user_func()
line 519 of /lib/eventslib.php: call to events_dispatch()
line 442 of /group/lib.php: call to events_trigger()
line 73 of /group/delete.php: call to groups_delete_group()