MDL-22062 Grade item's idnumber can be added from the calculation form
MDL-22062 Grade item's idnumber can be added from the calculation form

Before this patch, if activity module used multiply grade items (as
workshop does), the method add_idnumber() returned false because it
required empty idnumber in course_modules. The patch makes this check
only for grade_items with itemnumber 0.

dilawar/moodle