[Bug Report] El option "isObject" vue warn

### Element UI version
2.15.6

### OS/Browsers version
93.0.4577.82

### Vue version
2.6.12

### Reproduction Link
www.jsfiddle.com

### Steps to reproduce
Create a El Option select component with a computed list.

### What is Expected?
To not see the vue warn "vue.runtime.esm.js:619 [Vue warn]: Computed property "isObject" was assigned to but it has no setter."

### What is actually happening?
Creating a dropdown with El Option should not cause a vue warn to the system