Mybatisplus查询返回null但条数正确是什么原因？

public interface StudentMapper extends BaseMapper<Student> {
}

@RequestMapping("/testmybatisplus")
    @ResponseBody
    public void testmybatisplus() {
        System.out.println("===1===");
        List<Student> list=studentMapper.selectList(null);
        //Assert.isTrue(5 == list.size(), "");
        list.forEach(System.out::println);
    }

CREATE TABLE student(
   stu_id VARCHAR(50),
   stu_name VARCHAR(30),
   stu_sex VARCHAR(2),
   stu_age VARCHAR(4),
   stu_addr VARCHAR(50),
   stu_pwd VARCHAR(50)
)DEFAULT CHARSET=utf8;

@Data
@AllArgsConstructor
@NoArgsConstructor
public class Student {
    private String stu_id;
    private String stu_name;
    private String stu_sex;
    private String stu_age;
    private String stu_addr;
    private String stu_pwd;

}

mybatis-plus:
  configuration:
    log-impl: org.apache.ibatis.logging.stdout.StdOutImpl
    map-underscore-to-camel-case: false
  # 如果是放在src/main/java目录下 classpath:/com/*/*/mapper/*Mapper.xml
  # 如果是放在resource目录 classpath:/mapper/**.xml
  #mapper-locations: classpath:/mapper/**.xml