### Why is this Change Important & Necessary?

Fixes #12544.

The Rainbow Grades customization page crashes for some older courses
when legacy `gui_customization.json` data contains:

- bucket types not present in the current syllabus bucket list
- `"ids": null` instead of an array

The backend assumes these values are arrays and directly calls
`array_filter()` or iterates using `foreach`, resulting in a PHP 8
`TypeError`.

---

### What is the New Behavior?

- Bucket gradeable data is normalized to an empty array if missing or
invalid
- `json_bucket->ids` is normalized before iteration
- Legacy bucket metadata is initialized when needed

Malformed or legacy data is now treated as an empty bucket instead of
causing a crash. Existing valid behavior is unchanged.

#### Before (bug)

Legacy customization data with `"ids": null` causes a crash on the
customization page:

<img width="1470" height="956" alt="Screenshot 2026-04-01 at 10 52 16"
src="https://github.com/user-attachments/assets/1a9af415-d8c7-4a53-bee6-8bd8fb0ad64e"
/>

#### After (fixed)

The same input no longer crashes and the page loads successfully:

<img width="1470" height="956" alt="Screenshot 2026-04-01 at 10 54 29"
src="https://github.com/user-attachments/assets/b2267620-7510-4009-9f1b-daeb52acccda"
/>

---

### What steps should a reviewer take to reproduce or test the bug or
new feature?

1. Create or edit `rainbow_grades/gui_customization.json` in a course
directory
2. Add a legacy bucket entry such as:

```json
{
  "gradeables": [
    {
      "type": "legacy-bucket",
      "count": 1,
      "remove_lowest": 0,
      "percent": 0.25,
      "ids": null
    }
  ]
}
```

3. Visit:
   `/courses/<term>/<course>/reports/rainbow_grades_customization`

4. Before this patch:
   - Page crashes with:
`array_filter(): Argument #1 ($array) must be of type array, null given`

5. After this patch:
   - Page loads successfully and treats the malformed bucket as empty

---

### Automated Testing & Documentation

- Added regression test:
  `tests/app/models/RainbowCustomizationTester.php`

- Ran:
- `php vendor/bin/phpunit
tests/app/models/RainbowCustomizationTester.php`
- `php vendor/bin/phpunit
tests/app/controllers/admin/ReportControllerTester.php`

- No documentation changes required

---

### Other information

- Not a breaking change
- No migrations required
- No security concerns
- Supersedes abandoned PR #12545

---------

Co-authored-by: D'Andre Collins <[email protected]>