[Bug Report] picker级联时 第一个分类第一条数据没有子分类时 剩下的列将不显示

### VUX version
2.9.2

### OS/Browsers version
macOS 10.13.6

### Vue version
2.5.17

### Code

```html

import { Cell, Group, Picker, GroupTitle, XButton } from "vux";

let years = [];
for (var i = 2000; i <= 2030; i ) {
years.push({
name: i "年",
value: i ""
});
}
export default {
components: {
Picker,
GroupTitle,
XButton,
Cell,
Group
},
data() {
return {
year7: [
{
name: "中国",
value: "china",
parent: "0" // 为一级时可以不写 parent，但是此时允许为数字 0、空字符串或者字符串 '0'
},
{
name: "美国",
value: "USA",
parent: "0"
},
{
name: "美国001",
value: "usa001",
parent: "USA"
},
{
name: "美国002",
value: "usa002",
parent: "USA"
}
],
year7Value: []
};
}
};

```

### Steps to reproduce
.

### What is Expected?
显示所有列 如果第一列没有子项目 后面的列显示空

### What is actually happening?
后面的列不显示 即使滑到有子选项的列