Clarify TreeStore object-root probe fast-path comment

Agent-Logs-Url: https://github.com/Blosc/python-blosc2/sessions/973a79eb-d0c3-43ac-8008-54a9bd392be0

Co-authored-by: FrancescAlted <314521+FrancescAlted@users.noreply.github.com>

Author: Copilot

src/blosc2/tree_store.py

@@ -260,7 +260,7 @@ def _known_object_roots(self) -> set:
             return set(self._known_object_roots_cache)
 
         registered = self._object_roots()
-        # Fast path: when registry exists, avoid costly full-store probing.
+        # Fast path: when registry is non-empty, avoid costly full-store probing.
         roots = registered if registered else self._probed_object_roots()
         self._known_object_roots_cache = set(roots)
         return set(roots)