Fix hierarchy widget feature flag check

What does this MR do and why?

Fixes hierarchy widget feature flag check

How to set up and validate locally

1. Enable the work_items feature flag only for a group that has a project with issues/work_items.
2. For one of those issues/work_items, find the ID
3. Make sure the work item has a work_item_parent or work_item_childrent to fetch
4. Perform a query like

query fetchWorkItem {
  workItem(id: "gid://gitlab/WorkItem/<work_item_id>") {
    id
    widgets {
      type
      ... on WorkItemWidgetHierarchy {
      	parent {
          id
        }
        children {
          nodes {
            id
          }
        }
      }
    }
  }
}

Before this fix you would get a null children or parent fields. Now you should get the correct values in the response

MR acceptance checklist

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

Related to #362206 (closed)