Fix pipeline trigger via API fails with 500 Internal Server Error in 9.5.1

What does this MR do?

This MR fixes pipeline trigger via API fails with 500 Internal Server Error in 9.5.1.

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

V3 is almost going to have this regression too at https://gitlab.com/gitlab-org/gitlab-ce/issues/35650. Good to know earlier before we ship it.

Why was this MR needed?

This is regression. Currently, all users who have legacy triggers encounter this error.

Screenshots (if relevant)

N/A

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

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/37040