Empty stacktrace is causing an issue on server-side

*Error:* ``` object of type 'NoneType' has no len() ```
*file:* bugsense.js
*method:* generateExceptionData
*line:* 1371

```js
'backtrace': stacktrace
```
should be replaced by a safe statement, like
```js
'backtrace': (stacktrace && stacktrace.length) ? stacktrace : []
```