code: private static String mappingKey(Map<String, String> mapping, String fieldName) {
  		if (MapUtil.isEmpty(mapping)) {
  			return fieldName;
  		}
  		return ObjectUtil.defaultIfNull(mapping.get(fieldName), fieldName);
  	}
docs: 获取指定字段名对应的映射值
  
  @param mapping 反向映射Map
  @param fieldName 字段名
  @return 映射值，无对应值返回字段名
  @since 4.1.10