Fix OpenAPI V2 document generation

What does this MR do and why?

The changes introduced in !102241 (merged) let to message: 500 Internal Server Error after running rake gitlab:openapi:generate task.

Screenshots or screen recordings

Before

---
message: 500 Internal Server Error

After

---
info:
  title: GitLab API
  version: v4
swagger: '2.0'
produces:
- application/json
securityDefinitions:
  access_token_header:
    type: apiKey
    name: PRIVATE-TOKEN
    in: header
  access_token_query:
    type: apiKey
    name: private_token
    in: query
host: gitlab.com

How to set up and validate locally

1. Checkout the branch
2. Run gitlab:openapi:generate
3. Open openapi_v2.yaml file and validate there is no 500 Internal Server Error

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.