[Bug Report] First-time rendering of select-dropdown fails using select option group with multiple and filterable.

### Element UI version
2.13.0

### OS/Browsers version
Windows 10.0 / Chrome 80.0.3987.132

### Vue version
2.6.11

### Reproduction Link
https://jsfiddle.net/mmx38qxw/

### Steps to reproduce
1.Send request for data and will use it in vue later.

2.Get select option group with multiple and filterable ready. Use v-for to render `` in ``, applying data mentioned in 1.

3.Wait util request returns.

4.Click select input.

### What is Expected?
After the data returns and click, select-dropdown shold be rendered with all options revealing corresponding data no matter whether it's the first click.

### What is actually happening?
#### # For the first click
**First-time rendering** of select-dropdown fails and the dropdown box has an extremely small height, displaying nothing.
Here's the component structure in Vue Devtools when the problem appears. Correct component structure can be observed.

#### # For the Subsequent click
**Subsequent click** events applied to select-input will lead to correct behaviors.

This problem appears every first time that I click the select input after the data is ready. What's more, if I remove filterable or multiple, the components behave just right. So I think there might be some conflicts between multiple and filterable when using asynchronous data.

I try to 'imitate' click when the data returns to avoid the first-time rendering and it turns out to behave correctly. Using:
```
this.$refs.select.focus()
this.$nextTick(function() {
this.$refs.select.blur()
})
```