message: Add empty() method
diff: @@ -216,6 +216,9 @@ class ref_iterator : public std::iterator<
  return container_.size();
  }
  + bool empty() const {
  + return container_.empty();
  + }
  typename std::enable_if<!std::is_const<ref_t>::value, remove_const_t<ref_t>>::type
  operator*() {
  @@ -442,6 +445,11 @@ class filter_iterator : public std::iterator<
  }
  + bool empty() const {
  + return size() == 0;
  + }
  +
  +
  bool operator==(const filter_iterator& other) const {
  return (container_.size() == other.container_.size() && distance_ == other.distance_);
  }
  