Fix 500 when a type-changing quick action drops custom fields

What does this MR do and why?

Fix 500 when a type-changing quick action drops custom fields (#624496 (closed))

Changelog: fixed

References

• #624496 (closed)

Screenshots or screen recordings

How to set up and validate locally

1. Go to http://127.0.0.1:3000/-/graphql-explorer
2. Test that the following mutation works successfully

mutation {
  workItemCreate(input: {
    projectPath: "<project in an Ultimate group>",
    title: "repro",
    workItemTypeId: "gid://gitlab/WorkItems::Type/1",
    descriptionWidget: { description: "/promote_to_incident" },
    customFieldsWidget: []
  }) { workItem { id } errors }
}

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #624496 (closed)