Add check for notification to avoid il8n error in CoreBase

[sairina]

Summary

Presently, there is an existing error in CoreBase around UpdateNotification with the mostRecentNotification computed prop returning undefined and rendering an error because no notification existed. This error was discovered when writing tests for NotificationsRoot, and can be seen here through use of Vue dev tools and being able to log this error to the console - otherwise, this fails silently.

Note: any changes here will also need to be reflected in this PR: #9128. That PR is targeted for 0.16, but it might make sense for us to fix this in 0.15.

References

Reviewer guidance

• Please make sure this does not affect other parts of the codebase where we do have notifications, thanks!

Testing checklist

• Contributor has fully tested the PR manually
• If there are any front-end changes, before/after screenshots are included
• Critical user journeys are covered by Gherkin stories
• Critical and brittle code paths are covered by unit tests

PR process

• PR has the correct target branch and milestone
• PR has 'needs review' or 'work-in-progress' label
• If PR is ready for review, a reviewer has been added. (Don't use 'Assignees')
• If this is an important user-facing change, PR or related issue has a 'changelog' label
• If this includes an internal dependency change, a link to the diff is provided

Reviewer checklist

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md

[rtibbles]

Simple defensive programming guard. Very good.