Fix mypy violations in test_taskinstance.py (#59863)

This fixes: Item "None" of "Any | None" has no attribute  "id" (or "state") [union-attr]

Author: Dev-iL

airflow-core/tests/unit/models/test_taskinstance.py

@@ -2787,6 +2787,7 @@ def test_task_instance_history_with_hitl_history_is_created_when_ti_goes_for_ret
         with pytest.raises(TypeError):
             ti.run()
         ti = session.scalar(select(TaskInstance))
+        assert ti is not None
         # the ti.id should be different from the previous one
         assert ti.id != try_id
         assert ti.state == State.UP_FOR_RETRY