Fix vulnerability list footer spacing issue when grouped

Summary

When a group is expanded, the vulnerability list footer collides with the next group:

Steps to reproduce

1. Enable feature flag: echo "Feature.enable(:vulnerability_report_grouping)" | rails c
2. Fork https://gitlab.com/gitlab-examples/security/security-reports/
3. Create a pipeline against the default branch
4. Go to Secure > Vulnerability Report
5. Use the Group By Button
6. Check the footer