It is only necessary when there are (non-preview) attempts. However,
after making this change we have to ensure users cannot start attempts
when ->grade > 0 and ->sumgrades = 0.
Display ->grade on the order and paging tab of edit.php, so it is clear
what is going on when you are using that tab.
Finally, improve the error message a student gets if they try to start a
quiz with no questions.