code: public static <T> FlowableTransformer<T, T> doOnEmpty(final Action action) {
          return new FlowableTransformer<T, T>() {
  
              @Override
              public Publisher<T> apply(Flowable<T> upstream) {
                  return new FlowableDoOnEmpty<T>(upstream, action);
              }
          };
      }
docs: Returns a transformer that when a stream is empty runs the given
  {@link Action}.
  
  <p>
  <img src=
  "https://raw.githubusercontent.com/davidmoten/rxjava2-extras/master/src/docs/doOnEmpty.png"
  alt="image">
  
  @param action
  to be called when the stream is determined to be empty.
  @param <T>
  item type
  
  @return a transformer that when a stream is empty runs the given action.