[Bug Report] Since 2.12 SSR is broken

### 选择要提交 issue 的库
vue-easytable

### Issue 类型
Bug

### Issue 标题
Since 2.12 SSR is broken

### 仓库版本
2.12.0

### Vue 版本
2.6.14

### 浏览器
Firefox

### 系统类型
Linux

### 重现链接
render with Nuxt SSR

### 重现步骤
Because there are no globals (**document**) in SSR mode vue-easytable can no longer be rendered on server side (I am therefore stuck with version 2.11.0 that still works)

### 期望的结果是什么？
The page should render in SSR

### 实际的结果是什么？
Fatal error during render