我如何检查created_in列是否为none或不在触发器内?

不需要触发，会话严格模式就足够了:

set session sql_mode = '';

CREATE TABLE `users` (
`created_in` date NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
insert into users values ('');
select * from users;

| created_in ||:--------- || 0000-00-00 |

set session sql_mode = 'STRICT_ALL_TABLES';

insert into users values ('');

错误的日期值:" for column 'created_in' at row 1

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