Is there an existing issue for this?

• I have searched the existing issues

Description of the bug

When running git-cliff as part of our CI/CD pipeline, a panic occurs if the description field in a merge request’s metadata is null instead of an empty string.

thread 'main' (351) panicked at git-cliff-core/src/changelog.rs:590:18:
Could not get gitlab metadata: JsonError(Error("invalid type: null, expected a string", line: 1, column: 370937))

This issue started occurring after upgrading from git-cliff 2.10.1 to 2.11. It is still present in 2.12.

Steps To Reproduce

1. Have a merge request in GitLab where "description": null
2. Run git-cliff

Example JSON:

{
  "id": 319131,
  "iid": 149,
  "project_id": 19011,
  "title": "MR Title",
  "description": null,
  ...
}

Expected behavior

The description field should be treated as optional if it is null.

Screenshots / Logs

No response

Software information

• Operating system:
• Rust version:
• Project version: git-cliff 2.11 and 2.12 (issue occurs), 2.10.1 (working)

Additional context

No response