fix: use restriction_attributes property instead of private _restriction_attributes

FreeTable._restriction_attributes is None by default. The property
accessor initializes it to set() on first access. The make_condition
call in part_integrity="cascade" upward propagation was using the
private attribute directly, causing AttributeError when columns=None.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: dimitri-yatsenko

src/datajoint/diagram.py

@@ -518,7 +518,7 @@ def _propagate_restrictions(self, start_node, mode, part_integrity="enforce"):
                                 master_restr = make_condition(
                                     master_ft,
                                     (master_ft.proj() & child_ft.proj()).to_arrays(),
-                                    master_ft._restriction_attributes,
+                                    master_ft.restriction_attributes,
                                 )
                                 restrictions[master_name] = [master_restr]
                                 self._restriction_attrs[master_name] = set()