Prevent possible error on work items without children

What does this MR do and why?

Prevent possible error on work items without children

When loading a Requirement on work item page, this could previously null-ref. If this.workItem was defined, but findHierarchyWidgetChildren returned null

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Create a requirement
2. Load in /work_items/iid route

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.