Guard against undefined data in blob viewer Apollo handlers

What does this MR do and why?

Adds guard against undefined data in two places in blob viewer:

1. result({ data }) hook for the blobInfo query can be called with undefined data. according to https://gitlab.com/gitlab-org/gitlab/-/work_items/625054, this error hit 114,415 events affecting 110,907 users in approximately one day
2. adding fallback { } value in update hook

References

• Sentry-mirrored work item: https://gitlab.com/gitlab-org/gitlab/-/work_items/625054
• Sentry issue GITLABCOM-CLIENTSIDE-2M1B3: https://new-sentry.gitlab.net/organizations/gitlab/issues/3882965/

Screenshots or screen recordings

No visible UI change — this is a defensive guard only. The table below shows that blob pages continue to work correctly after the fix (rich viewer README, source toggle, JS file with syntax highlighting, line-hash navigation, blame view).

Before	After
No visual change (defensive guard)

How to set up and validate locally

1. Start GDK.
2. Visit a blob page, for example http://gdk.test:3000/flightjs/Flight/-/blob/master/README.md.
3. Toggle between Code and Preview views and confirm the content renders without errors.
4. Visit a code file (e.g. a .js file) and verify syntax highlighting works.
5. Append #L20 to the URL and confirm the page scrolls to and highlights the correct line.
6. Open the Blame view and confirm it loads correctly.
7. Check the browser console — confirm no TypeError: Cannot read properties of undefined (reading 'project') appears.
8. Run the unit tests:

yarn jest spec/frontend/repository/components/blob_content_viewer_spec.js

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.