fix crash when BrowserTypes match nothing from navigator.userAgent .

When BrowserTypes match nothing from navigator.userAgent such as open a webview from AdobeAIR runtimes , the userAgent is
 ``` 
mozilla/5.0 (macintosh; u; intel mac os x; zh-hans) applewebkit/533.19.4 (khtml, like gecko) adobeair/3.8 '  
```

now the browserType is null and will crash on 
```
if ( browserTypes.length > 0 ) {   //browserTypes = null
```