Forkjoin与空(或非)可观察数组

我正试图检测我所有的观察结果何时完成.我有以下Observables:

let observables:any[] = [];
if(valid){
    observables.push(new Observable((observer:any) => {
        async(()=>{
            observer.next();
            observer.complete();
        })
    }))
}
if(confirmed){
    observables.push(new Observable((observer:any) => {
        async(()=>{
            observer.next();
            observer.complete();
        })
    }))
}

Observable.forkJoin(observables).subscribe(
    data => {
        console.log('all completed');
    },
    error => {
        console.log(error);
    }
);

Rx.Observable.forkJoin([]).subscribe(
    val => {
        console.log('next');
    },
    err => {
        console.log('err');
    },
    () => {
        console.log('complete')
    }
);