Fix tree renderer default sort behavior

- Change effective_sort logic from `if sort_spec is not None` to `if sort_spec`
- This ensures both None and empty string trigger default mtime descending sort (-d)
- Aligns tree renderer with flat renderer behavior
- Fixes test_tree_sibling_sort_by_name_reverse which expects default mtime descending

🤖 Generated with Claude Code

Co-Authored-By: Claude <noreply@anthropic.com>

Author: elifarley

src/cedarmapper/render/tree.py

@@ -51,7 +51,7 @@ def render_tree(
 
     # prepare comparator: default to '-d' (mtime desc) if user didn't supply a sort
     comparator = None
-    effective_sort = sort_spec if sort_spec is not None else "-d"
+    effective_sort = sort_spec if sort_spec else "-d"
     specs = parse_sort_spec(effective_sort)
     if specs:
         comparator = make_comparator(specs)