[bug report] when the trigger mode of Popover component is manual and the initial value of V-model is true, an exception will be displayed

### Element UI version
2.15.1

### OS/Browsers version
win10/chrom:89.0.4389.82

### Vue version
2.6.10

### Reproduction Link
https://codepen.io/yimijianfang/pen/PobeJxZ

### Steps to reproduce
初始值为true不能正常初始化

### What is Expected?
触发方式为manual且v-model为true时正常显示

### What is actually happening?
初始值为true不能正常初始化