Fix issuable sidebar icon of notification disabled

What does this MR do?

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30961 introduced a feature of disabling notifications at the project/group level. Unfortunately this left a visual regression where there was no handling of when the issuable sidebar is collapsed.

Screenshots

Before	After

Note	Image
No changes to uncollapsed view

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content