fix the STR_TO_DATE incompatible between MySQL and TiDB (#12623)

[lonng]

cherry-pick #12623 to release-2.1

---

Signed-off-by: Lonng heng@lonng.org

What problem does this PR solve?

The result of select str_to_date('20190101','%Y%m%d%H%i%s') is incompitable between MySQL and TiDB.

MySQL

mysql> select str_to_date('20190101','%Y%m%d%H%i%s');
+----------------------------------------+
| str_to_date('20190101','%Y%m%d%H%i%s') |
+----------------------------------------+
| 2019-01-01 00:00:00                    |
+----------------------------------------+
1 row in set (0.01 sec)

TiDB

mysql> select str_to_date('20190101','%Y%m%d%H%i%s');
+----------------------------------------+
| str_to_date('20190101','%Y%m%d%H%i%s') |
+----------------------------------------+
| NULL                                   |
+----------------------------------------+
1 row in set (0.01 sec)

What is changed and how it works?

Fix it.

Check List

Tests

• Unit test

Release note

• fix the STR_TO_DATE incompatible between MySQL and TiDB

[lonng]

/run-all-tests

[AilinKid]

LGTM

[tiancaiamao]

LGTM