[bug report] message component, when offset is set to 0, the offset is not effective, and the default value of 20 is still used

### Element UI version
2.15.6

### OS/Browsers version
Windows / Microsoft Edge 95.0.1020.30

### Vue version
2.6.12

### Reproduction Link
https://codepen.io/xs47968224/pen/gOxRXbE

### Steps to reproduce
this.$message({
message: '操作成功',
type: 'success',
offset: 0
})

### What is Expected?
期望消息弹框的上边距为0px

### What is actually happening?
实际消息弹框的上边距为20px