code: @CheckReturnValue
      @SchedulerSupport(SchedulerSupport.NONE)
      public final Single<T> toSingle(T defaultValue) {
          ObjectHelper.requireNonNull(defaultValue, "defaultValue is null");
          return RxJavaPlugins.onAssembly(new MaybeToSingle<T>(this, defaultValue));
      }
docs: Converts this Maybe into a Single instance composing disposal
  through and turning an empty Maybe into a Single that emits the given
  value through onSuccess.
  <dl>
  <dt><b>Scheduler:</b></dt>
  <dd>{@code toSingle} does not operate by default on a particular {@link Scheduler}.</dd>
  </dl>
  @param defaultValue the default item to signal in Single if this Maybe is empty
  @return the new Single instance