Issue description:

When a user is graded in a category using Natural aggregation, the category total will incorrectly show as a blank grade when using the following configuration:

1. Hidden Course total
2. 'Hide totals if they contain hidden items' set to 'Hide' in Course grade settings

The gradebook acts as if there is a hidden grade item in the category and displays a blank grade for the category total in the User report screen.

Replication steps:

1. Create a new course
2. Enrol a Student user into the course
3. Go to Grades tab
4. In the dropdown, select 'Course grade settings'
5. Ensure User report>Hide totals if they contain hidden items is set to 'Hide'
6. Save changes
7. In the dropdown, select 'Gradebook setup'
8. Add a Category 'Test category' and ensure 'Aggregation' is set to Natural
9. Add a Grade item 'Test 1' and ensure the Parent category is 'Test category'
10. Hide the Course total
11. In the dropdown, select 'Grader report'
12. Add a grade of '100' for the Student user
13. In the dropdown, select 'User report'
14. Search for the enrolled Student user
15. Observe the value for 'Test category'

Observed behaviour:

'Test category' displays a blank grade for the user

Expected behaviour:

'Test category' displays '100' for the user