Angular Material, Autocomplete loading indicator and showing text (no results found)

#### Feature Description

I need to show loading when searching for data and also show text when no record is found.
I have the following html excerpt:
```html

arrow_drop_down

{{
index !== to.filterKeys.length - 1
? value[ky] + ' - '
: value[ky]
}}


{{ value }}

```

Component.ts code:
```javascript
ngOnInit(): void {
super.ngOnInit();

this._unsubscribeAll = new Subject();
this.isLoading = false;

this.result$ = this.formControl.valueChanges.pipe(
takeUntil(this._unsubscribeAll),
debounceTime(300),
startWith(''),
tap(() => {
this.isLoading = true
}),
switchMap(term => this.to.filter(term))
);
}
```

for loading, I would control it with the `tap()` method, but my problem is how to know that the request inside `switchMap(term => this.to.filter(term))` was finished, so I set the loading variable to false?

my second problem is how to show a registration message not found, when the server returns an empty array, because I am working with async.

从提供的 Angular 模板和 ngOnInit 中的 valueChanges 管道开始，然后查看 Angular Material autocomplete 文档及相关示例。当 autocomplete 能够明确显示正在进行搜索，并且在异步结果为空时显示无结果消息时，即视为完成；未指定任何 repository 文件或测试。

由索引模型根据 Issue 内容生成。