Resolve "[Rails5] Fix spec/models/ci/pipeline_spec.rb"

What does this MR do?

In Rails 5 enum returns value instead of key.
For this case, the NilClass is returned instead of unknown which breaks validation of the source attribute.

This MR adds a custom validator that returns the correct result for both rails4 and rails5.

Are there points in the code the reviewer needs to double check?

No.

Why was this MR needed?

Migration to Rails 5.0.

Screenshots (if relevant)

No.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

#14286 (closed)

Closes #46229 (closed)