Fix bug in cargo parent path detection

[Iron-56]

Fixes #1436

Summary

Fix crash when resource.parent(codebase) returns None in Rust plugin during Cargo workspace detection.

Tasks

• Reviewed contribution guidelines
• PR is descriptively titled 📑 and links the original issue above 🔗
• Tests pass -- look for a green checkbox ✔️ a few minutes after opening your PR
  Run tests locally to check for errors.
• Commits are in uniquely-named feature branch and has no merge conflicts 📁
• Updated documentation pages (not applicable)
• Updated CHANGELOG.rst (applicable)