cleanup code

lib/pages/rates/shipment_rates.dart

@@ -200,7 +200,7 @@ class _ShipmentRatesState extends State<ShipmentRates> {
   }
 
   List<Widget> getDiscountWidget(List<DiscountByWeight> discounts) {
-    if (discounts == null) return [];
+    if (discounts.isEmpty) return [];
     return discounts.map((d) {
       return Container(
         child: _row("${d.weight.toStringAsFixed(2)} lb",