return immediately if source response empty array. otherwise adding some rx functions.

Emptiable.ts

// return immediately empty if source response empty array.
// otherwise adding some functions.
const Emptialbe = <T>(source: Observable<T[]>, additional: Observable<T[]>) => {
  source.pipe(
    switchMap(array => {
      if (array.length === 0) {
        return of(array)
      } else {
        return additional(array)
      }
    })
  )
}

// example
const someApiResponse$ = someApi();
const anotherApiResponse$ = anotherApi();

Emptialbe<string>(apiResponse$, (array) => forkJoin(of(array), anotherApiResponse$)
  .pipe(
    map([someApiResponse, anotherApiResponse] => {
      return mappedValue;
    })
  )
)