修改null 判断逻辑，防止空字符串的情况。review by 张文文。

@@ -125,7 +125,7 @@ class GroupPurchaseContainer extends Component {
         }
 
         let miniProgramPath;
-        if (activityId == null) {
+        if (!activityId) {
           miniProgramPath = '/pages/group/list?';
         } else {
            miniProgramPath = '/pages/group/list?activityId=' + activityId;

@@ -16,7 +16,7 @@ export default class groupPurchaseService {
     async fetchGroupProductList(activityId, page=1, limit=20, fromPage, listParams) {
 
         let bodyParam;
-        if (activityId == null) {
+        if (!activityId) {
           bodyParam = {
               method: 'app.collage.productList.page',
               ...listParams,