【报Bug】嵌套生成时使用过滤器报错

当绑定对象嵌套生成视图时使秀过滤器报错。

错误如下：
Cannot read property 'map' of undefined; [Component] Lifetime Method Error @ pages/test/test#attached
TypeError: Cannot read property 'map' of undefined
at Proxy.render (http://127.0.0.1:51017/appservice/pages/test/test.js:160:30)
at VueComponent.Vue._render (http://127.0.0.1:51017/appservice/common/vendor.js:6803:28)
at VueComponent.updateComponent (http://127.0.0.1:51017/appservice/common/vendor.js:8749:25)
at Watcher.get (http://127.0.0.1:51017/appservice/common/vendor.js:7590:31)
at new Watcher (http://127.0.0.1:51017/appservice/common/vendor.js:7579:51)
at mountComponent$1 (http://127.0.0.1:51017/appservice/common/vendor.js:8755:9)
at VueComponent.Vue.$mount (http://127.0.0.1:51017/appservice/common/vendor.js:9011:16)
at H.attached (http://127.0.0.1:51017/appservice/common/vendor.js:3526:22)
at r.safeCallback (http://127.0.0.1:51017/appservice/dev/WAService.js:1:818220)
at r.call (http://127.0.0.1:51017/appservice/dev/WAService.js:1:818113)

报错部分生成的代码：

var render = function render() {  
      var _vm = this;  
      var _h = _vm.$createElement;  
      var _c = _vm._self._c || _h;  
      var l1 = _vm.cart.shops.map(function (shop, i) {  
        var l0 = shop.products.map(function (item, j) {  
          var f0 = _vm._f("fmNum2")(item.price);  

          return {  
            $orig: _vm.__get_orig(item),  
            f0: f0  
          };  
        });  
        return {  
          $orig: _vm.__get_orig(shop),  
          l0: l0  
        };  
      });  
      _vm.$mp.data = Object.assign({}, {  
        $root: {  
          l1: l1  
        }  
      });  
    };

生成的代码部分cart.shops实际上在数据没加载前是没有的，这样打开页面的时候就调用了render导致报上面那个错了

重现问题代码：

<template>  
    <view>  
        <button @click="loadData">加载数据</button>  
        <view v-for="(shop,i) in cart.shops" :key="i">  
            <view>{{shop.name}}</view>  
            <view>  
                <view v-for="(item,j) in shop.products" :key="j">  
                    <text>{{item.name}}</text>  
                    <text>{{item.price|fmNum2}}</text>  
                </view>  
            </view>  
        </view>  
    </view>  
</template>  

<script>  
    export default {  
        data() {  
            return {  
                cart: {}  
            }  
        },  
        filters: {  
            fmNum2(val) {  
                return val.toFixed(2);  
            }  
        },  
        methods: {  
            loadData() {  
                this.cart = {  
                    total: 1000,  
                    shops: [{  
                        name: '店铺1',  
                        products: [{  
                            name: '商品1-1',  
                            price: 100  
                        }, {  
                            name: '商品1-2',  
                            price: 200  
                        }]  
                    }, {  
                        name: '店铺2',  
                        products: [{  
                            name: '商品2-1',  
                            price: 100  
                        }, {  
                            name: '商品2-2',  
                            price: 200  
                        }]  
                    }]  
                };  
            }  
        }  
    }  
</script>  

<style>  
</style>