Resolve "Project variables validate without any scopes disregarding environment_scope"

What does this MR do?

EE port in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4510

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #43191 (closed)