code: public static <T, C extends Collection<T>> Stream<T>
      ifExistIntoStream(C collection) {
          return getOptional(collection).map(Collection::stream)
                  .orElseGet(Stream::empty);
      }
docs: If exist into stream stream.
  
  @param <T>        the type parameter
  @param <C>        the type parameter
  @param collection the collection
  @return the stream