select 组件在新值为 0 时没有触发 (modelChange) 事件

Issue Type:
- [x] Bug
- [ ] Feature
- [ ] Help

Version & Environment:  
[在线demo](https://stackblitz.com/edit/angular-ifbork)


Description:
select 选择器在选择 0 (number 类型) 时没有触发 (modelChange) 事件，导致这个问题的代码可能是 [L116](https://github.com/ElemeFE/element-angular/blob/master/src/select/select.ts#L116)

