[Bug Report] Select component displayed wrong when the options is empty but the v-model has the default value

### Element UI version
2.12.0

### OS/Browsers version
macOS 10.15.3 / Chrome 80.0.3987.149

### Vue version
2.6.10

### Reproduction Link
https://codepen.io/cactusnix/pen/MWwBVEJ

### Steps to reproduce
1. I use the select component to display the default item that selected by the user.
2. The el-options has the empty options list, but the v-model has the default value.
3. I wished the select component may show nothing, but it showed the value that is bound with v-model.

### What is Expected?
I wished the select component may show nothing, but it showed the value that is bound with v-model.

### What is actually happening?
I wished the select component may show nothing, but it showed the value that is bound with v-model.