Ensure targetdir is created for single dir passthroughs

What does this MR do?

Ensures that target directory is autogenerated in the presence of single dir passthroughs.

What are the relevant issue numbers?

Handle empty TargetDir in ruleset package (gitlab-org/gitlab#576155) • Julian Thome • 18.6

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 updated/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