Ensure vulnerability_statistic records are being created with valid traversal_ids

Summary

There is still at least one more spot in the code where invalid vulnerability_statistics records are being created (see here). We need to change this before we can safely add in the traversal_ids constraint in #518683 (closed).

Owners

• Team: groupsecurity infrastructure
• Most appropriate slack channel to reach out to: #g_srm_security_infrastructure
• Best individual to reach out to: @anarinesingh