fix: 运费金额为空时返回空值，便于顺丰批次展示月结

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/tests/test_sf_express_settlement.py

@@ -181,3 +181,33 @@ def test_freight_model_allows_null_amount_and_settlement_columns():
     assert cols["waybill_no"].nullable is True
     assert cols["settle_status"].nullable is True
     assert cols["settle_time"].nullable is True
+
+
+def test_amount_serializer_keeps_null_instead_of_zero():
+    from types import SimpleNamespace
+    from app.crud.sample_freight import _row_to_dict
+    from app.services.market_freight import _batch_dict
+
+    obj = SimpleNamespace(
+        id="f1",
+        freight_no="YF1",
+        transport_mode="sfkd",
+        freight_amount=None,
+        waybill_no=None,
+        settle_status="wjs",
+        settle_time=None,
+        tenant_no="zkxw",
+        order_num=0,
+        remark="",
+        create_by="u1",
+        create_time=None,
+        update_time=None,
+    )
+    admin = _row_to_dict(obj, transport_mode_label="顺丰快递", create_by_name="张三")
+    market = _batch_dict(obj, 1, "顺丰快递", "张三")
+    assert admin["freightAmount"] is None
+    assert admin["waybillNo"] == ""
+    assert admin["settleStatus"] == "wjs"
+    assert admin["freightAmountLabel"] == "月结"
+    assert market["freightAmount"] is None
+    assert market["freightAmountLabel"] == "月结"