code: public static <V> Map<String, V> cleanupMap(Map<String, V> map) {
          if (map == null || map.isEmpty()) {
              return null;
          }
  
          Map<String, V> result = new HashMap<String, V>(map.size());
          Set<Entry<String, V>> entries = map.entrySet();
  
          for (Entry<String, V> entry : entries) {
              if (entry.getValue() != null) {
                  result.put(entry.getKey(), entry.getValue());
              }
          }
  
          return result;
      }
docs: 清除字典中值为空的项。
  
  @param <V> 泛型
  @param map 待清除的字典
  @return 清除后的字典