[Bug Report] Slider中的List数据异步加载，无数据时也无法通过v-show隐藏该控件

### VUX version
2.9.4

### OS/Browsers version
Windows 10 / Chrome 74

### Vue version
2.6.10

### Code

```html
Template:

data:
banners: null

method: //异步获取banner
var self = this
axios.get('banners').then(data => {
self.banners = data.map(item => {
return { img: item.img, url: item.url, title: item.name }
})
})
```

### Steps to reproduce
按代码逻辑异步获取banner图片

### What is Expected?
1、有图的情况banners.length>0,显示slider组件
2、无图的情况，不显示silder组件

### What is actually happening?
不显示组件，但F12调试时发现:
1、banner取到了3条值，组件的html代码拼装有
2、vux-swiper的style有height:0px，如果我们手动改为100px，slider工作正常

Start with the provided Vue reproduction using , v-show, and asynchronously populated banners. Check how the slider handles its initial null or empty list and subsequent updates; done means a populated list renders at the expected height and an empty list remains hidden without breaking the component.

Scritto dal modello di indicizzazione a partire dal testo della issue.