[Bug Report] el-table组件设置type="selection"，当列表为空时，表头上的复选框虽然展示为禁用状态但还是可以选择
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- Dominant language
- Vue
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Description
### Element UI version
2.12.0
### OS/Browsers version
MacOS/Chrome
### Vue version
2.5.17
### Reproduction Link
https://jsfiddle.net/gjyp57o8/3/
### Steps to reproduce
一个空的列表，勾选表头复选框
### What is Expected?
不可以选择
### What is actually happening?
可以选择
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