Fix some issues in search

[rtibbles]

Summary

• Fixes issue where default thumbnail was returning an empty object not an empty string
• Fixes props passing for mobile views of the search panel

References

Follow up to #8646 (comment)
Follow up to #8548

Reviewer guidance

When viewing in mobile view with 'list' cards, does it avoid returning a weird 404 URL when a content node doesn't have a thumbnail?
In mobile view, can you open and use the search side panel?

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Testing checklist

• Contributor has fully tested the PR manually
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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
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