Resolve "API: PUT /projects/:id fails when only ci_config_path is specified"

What does this MR do?

Fix the failing /projects/:id API call when only ci_config_path is specified. See: https://gitlab.com/gitlab-org/gitlab-ce/issues/42509

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

I don't think so, but I'm just a newbie here.

Why was this MR needed?

Because the API failed when only the ci_config_path was specified in the /projects/:id call-

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-qa manual pipeline job)

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/42509

Closes #42509 (closed)