code: public static <T> Publisher<T> toPublisher(Single<T> single) {
          if (single == null) {
              throw new NullPointerException("single");
          }
          return new SingleAsPublisher<T>(single);
      }
docs: Converts a Single into a Publisher which emits an onNext+onComplete if
  the source Single signals a non-null onSuccess; or onError if the source signals
  onError(NullPointerException) or a null value.
  @param <T> the value type
  @param single the Single instance to convert
  @return the Publisher instance
  @since 1.1
  @throws NullPointerException if single is null