Don't parse gitleaks output when no leaks

What does this MR do?

Don't parse gitleaks output when exit status is 0.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/10257

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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  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer