Edge case with USS list of directories with items: undefined/null

Describe the bug

Review of another PR pointed out an edge case with USS list of directories with items: undefined/null and missing test coverage for edge case

• Issue: The new USS directory-detection heuristic is better than main for child-only extender listings, but still requires at least one returned item to classify as directory.
• a residual edge case for successful empty-directory responses (items: []) from extenders.
• Why it matters: Empty folders from some extenders may still fail to be cached/recognized consistently.
• The unresolved edge case can regress silently in future refactors.
• Suggested fix: Add an explicit empty-list fallback path for directory context (or a follow-up stat/lookup check) so items: [] can still be treated correctly when appropriate. Add one unit test for successful empty-list directory response from an extender compatibility scenario.

To Reproduce

Expected behavior

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