[Bug Report] picker 联动问题

### VUX version
2.9.1-rc.1

### OS/Browsers version
win10/chrome

### Vue version
2.5.17-beta.0

### Code

```html

import Vue from 'vue'
import {
XInput, Group, XButton, Cell, Checker, CheckerItem, PopupPicker, Datetime,
XSwitch, XTextarea, Toast, ConfirmPlugin
} from 'vux'
Vue.use(ConfirmPlugin)

export default {
components: {
XInput, XButton, Group, Cell, Checker, CheckerItem, PopupPicker, Datetime,
XSwitch, XTextarea, Toast
},
data() {
return {
value: {
SSQX: [],
SSQX1: [],
},
SSList: [{
name: '天津市',
value: 'tj',
parent: '0'
},{
name: '北京市',
value: 'bj',
parent: '0'
}, {
name: '和平区',
value: 'hpq',
parent: 'tj'
}, {
name: '河北区',
value: 'hbq',
parent: 'tj'
},],
SSList1: [{
name: '北京市',
value: 'bj',
parent: '0'
},{
name: '天津市',
value: 'tj',
parent: '0'
}, {
name: '和平区',
value: 'hpq',
parent: 'tj'
}, {
name: '河北区',
value: 'hbq',
parent: 'tj'
},],
}
},
methods: {
},
mounted() {
}
}

```

### Steps to reproduce
两个不同的联动list，只有首位对象有没有子数据不同，
天津市下有区县，显示正常，
北京市下无区县，显示就有异常，

### What is Expected?
无论list中第一个对象有没有子数据，都显示正常

### What is actually happening?
无论list中第一个对象有没有子数据，都显示正常