message: modify zpopmin zpopmax cmd return when empty
diff: @@ -895,7 +895,7 @@ void ZPopmaxCmd::DoInitial() {
  void ZPopmaxCmd::Do(std::shared_ptr<Partition> partition) {
  std::vector<blackwidow::ScoreMember> score_members;
  rocksdb::Status s = partition->db()->ZPopMax(key_, count_, &score_members);
  - if (s.ok()) {
  + if (s.ok() || s.IsNotFound()) {
  char buf[32];
  int64_t len;
  res_.AppendArrayLen(score_members.size() * 2);
  @@ -930,7 +930,7 @@ void ZPopminCmd::DoInitial() {
  void ZPopminCmd::Do(std::shared_ptr<Partition> partition) {
  std::vector<blackwidow::ScoreMember> score_members;
  rocksdb::Status s = partition->db()->ZPopMin(key_, count_, &score_members);
  - if (s.ok()) {
  + if (s.ok() || s.IsNotFound()) {
  char buf[32];
  int64_t len;
  res_.AppendArrayLen(score_members.size() * 2);
  