Angular -当使用异步管道时，如何检查* to中的数组长度？

import { Pipe, PipeTransform } from '@angular/core';
import { Observable } from 'rxjs';

@Pipe({
  name: 'arrayLength'
})
export class ArrayLengthPipe implements PipeTransform {
  transform(array$: Observable<any[]>): Observable<number> {
    return array$.pipe(
      map(array => array.length)
    );
  }
}

<p>数组长度为: {{ array$ | arrayLength | async }}</p>

<div *ngIf="(array$ | async)?.length > 0">
  数组不为空
</div>
<div *ngIf="(array$ | async)?.length === 0">
  数组为空
</div>