fix: don't render empty ul when folder has no children

[geeta102]

Issue:

Fixes # 4039

Demo:

Not applicable here as this is not a UI change. The fix removes
an empty <ul> from the DOM when a folder has
no children, which improves screen reader
experience but has no visual impact.

Changes:

Added children.length > 0 condition in
FileNode.jsx to prevent rendering empty
ul element when folder has no children

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• has no typecheck errors (npm run typecheck)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123
• meets the standards outlined in the accessibility guidelines