`vue.config.js` is broken after run `vue invoke`

### Version
3.0.0-alpha.8

### Steps to reproduce
1. `vue create test` and use default options
2. `cd test`
3. `vue invoke eslint` and use default options

### What is expected?
Nothing changed

### What is actually happening?
content of `vue.config.js` changed to `module.exports = undefined`