address type error in tests

tests/system/small/test_session.py

@@ -236,8 +236,13 @@ def test_read_gbq_w_anonymous_query_results_table(session: bigframes.Session):
 def test_read_gbq_w_primary_keys_table(
     session: bigframes.Session, usa_names_grouped_table: bigquery.Table
 ):
+    # Validate that the table we're querying has a primary key.
     table = usa_names_grouped_table
-    primary_keys = table.table_constraints.primary_key.columns
+    table_constraints = table.table_constraints
+    assert table_constraints is not None
+    primary_key = table_constraints.primary_key
+    assert primary_key is not None
+    primary_keys = primary_key.columns
     assert len(primary_keys) != 0
 
     df = session.read_gbq(f"{table.project}.{table.dataset_id}.{table.table_id}")