Use explicit fallback branch for object-root probing

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

@@ -261,7 +261,10 @@ def _known_object_roots(self) -> set:
 
         registered = self._object_roots()
         # Fast path: when registry is non-empty, avoid costly full-store probing.
-        roots = registered if registered else self._probed_object_roots()
+        if registered:
+            roots = registered
+        else:
+            roots = self._probed_object_roots()
         self._known_object_roots_cache = set(roots)
         return set(roots)