Fix optional arugments for POST :id/variables

What does this MR do?

Use declared_params(include_missing: false) to ignore optional arguments.

Why was this MR needed?

Previously there's no optional arguments in this API, therefore we're not using include_missing: false. Now we do, so we need it.

Fix regression introduced by https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11688

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests
  • Added for this feature/bug

What are the relevant issue numbers?

Closes #34282 (closed)