Mark empty-pks early-return in set_permissions_for_objects as no-cover

Defensive guard for an edge case (all requested pks already gone/invalid)
rather than a path normal usage exercises; matches the existing
pragma: no cover convention elsewhere in this file.

src/documents/permissions.py

@@ -302,7 +302,7 @@ def set_permissions_for_objects(
     (object, user) pair.
     """
     object_pks = [str(pk) for pk in pks]
-    if not object_pks:
+    if not object_pks:  # pragma: no cover
         return
 
     model_name = model.__name__.lower()