Add extra unit test

What does this MR do and why?

Adds a unit test for feature flag disabled case.

Screenshots or screen recordings

Nothing changed visually.

How to set up and validate locally

Run yarn jest ee/spec/frontend/security_dashboard/components/shared/vulnerability_report/vulnerability_report_spec.js

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #433099 (closed)