code: @SuppressWarnings("unchecked")
  	protected Map<String, Object> getNestedMap(Map<String, Object> map, String key) {
  		Object value = map.get(key);
  		if (value == null) {
  			return Collections.emptyMap();
  		}
  		return (Map<String, Object>) value;
  	}
docs: Return the nested map with the specified key or empty map if the specified map
  contains no mapping for the key.
  @param map the content
  @param key the key of a nested map
  @return the nested map