Fix updating group level work item labels

What does this MR do and why?

Now work items can belong directly to a group. We need to make just a few changes for updating labels to work in this scenario

How to set up and validate locally

1. Enable the feature flag in Rails console Feature.enable(:namespace_level_work_items)
2. Create a group level work item with the following query at http://127.0.0.1:3000/-/graphql-explorer so you can use it in the next query

   mutation {
      workItemCreate(input: {namespacePath: "flightjs", title: "test group level work item", workItemTypeId: "gid://gitlab/WorkItems::Type/1"}) {
        errors
        workItem {
          id
          iid
        }
      }
    }

3. Update labels on the newly created work item

    mutation {
      workItemUpdate(input:{id: "<id_returned_in_previous_step>", labelsWidget: {addLabelIds: ["gid://gitlab/GroupLabel/52", "gid://gitlab/GroupLabel/50"]}}) {
        errors
        workItem {
          id
          iid
          widgets {
            ... on WorkItemWidgetLabels {
              allowsScopedLabels
              labels {
                nodes {
                  color
                  id
                  title
                }
              }
            }
          }
        }
      }
    }

   Before this change, you will get and undefined method for nil error. Now in the response you should see the specified labels associated with the work item

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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

Related to #426472 (closed)