Fix empty vulnerability when bundler audit raises warning

What does this MR do?

Handle the case when bundle audit check raises a warning before listing vulnerabilities, or before showing No vulnerabilities. Prior to this MR, warnings result in extra vulnerabilities that are almost empty. The title of the extra vulnerability is Vulnerability in.

What are the relevant issue numbers?

gitlab-org/gitlab#347245 (closed)

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
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• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer