src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    @SuppressWarnings("unchecked")
    public static <T> Observable<T> empty() {
        return RxJavaPlugins.onAssembly((Observable<T>) ObservableEmpty.INSTANCE);
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    @SuppressWarnings("unchecked")
    public static <T> Observable<T> empty() {
        return RxJavaPlugins.onAssembly((Observable<T>) ObservableEmpty.INSTANCE);
    }
Returns an Observable that emits no items to the {@link Observer} and immediately invokes its
{@link Observer#onComplete onComplete} method.
<p>
<img width="640" height="190" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/empty.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code empty} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>

@param <T>
the type of the items (ostensibly) emitted by the ObservableSource
@return an Observable that emits no items to the {@link Observer} but immediately invokes the
{@link Observer}'s {@link Observer#onComplete() onComplete} method
@see <a href="http://reactivex.io/documentation/operators/empty-never-throw.html">ReactiveX operators documentation: Empty</a>