Fix unguarded codequalityReportsComparer.status access and nil pipeline guard

What and why

This MR addresses issue #594017.

When a merge request has no diff_head_pipeline, the backend resolver returns null and the frontend stop-polling condition throws a TypeError because it accesses codequalityReportsComparer.status without guarding against null. This MR fixes the crash by using optional chaining / nil-safe access in the stop-polling logic.

What this MR changes

Files updated:

• app/assets/javascripts/diffs/components/app.vue
• app/assets/javascripts/groups/show/components/app.vue
• app/assets/javascripts/groups_projects/components/tabs_with_list.vue
• spec/frontend/groups_projects/components/tabs_with_list_spec.js

Implementation notes

• Updated the stop-polling condition(s) to safely handle codequalityReportsComparer === null (nil-safe access with optional chaining), so the UI keeps polling instead of throwing.
• Added/updated a regression spec around the behavior covered by tabs_with_list_spec.js.

How to verify

1. Review the changed stop-polling condition(s) in the Vue components to confirm all accesses to codequalityReportsComparer are nil-safe.
2. Run the relevant frontend/unit tests for the touched area (includes spec/frontend/groups_projects/components/tabs_with_list_spec.js) and ensure they pass.
3. Confirm the MR pipeline is green in GitLab.

Contribution standards

This MR follows GitLab’s contributor guidance for merge requests:

• Merge request workflow
• Code review guidelines

Closes #594017