Fix nil pointer deref

What does this MR do?

Fix nil pointer dereference. Incorporate the new VET release that includes the fix: https://gitlab.com/gitlab-org/security-products/vet/vet/-/tags/v0.15.1

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/security-products/vet/vet/-/issues/75

Closes gitlab-org/gitlab#371677 (closed)

Demo project:

1. reproduction of the vulnerability in a test project: https://gitlab.com/julianthome/vettest/-/jobs/2929918890
2. MR that includes the fix (docker image produced in the context of this MR): https://gitlab.com/julianthome/vettest/-/jobs/2934048457

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer