调整js判断

public/js/change_order.js

@@ -454,7 +454,7 @@
                                 change_str += goods_name+'单价不能为0或空 ';
                             }
 
-                            if (order_goods_type == 1 && buyer_id == '') {
+                            if (order_goods_type == 1 && buyer_id == null) {
                                 change_buyer_id = true;
                                 change_str += goods_name+'采购员不能为空 ';
                             }