The function of if with enum type is not same with mysql

[lilinghai]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

create table t(a int,b enum("b","y","1"));
insert into t values(0,"y"),(1,"b"),(null,null),(2,"1");
SELECT count(*) FROM t where if(a,b ,null);

2. What did you expect to see? (Required)

2

3. What did you see instead (Required)

1
the result of mysql 8.0.24 is 2

4. What is your TiDB version? (Required)

Release Version: v4.0.0-beta.2-2807-g289dcfefd-dirty
Edition: Community
Git Commit Hash: 289dcfe
Git Branch: master
UTC Build Time: 2021-05-06 15:11:16
GoVersion: go1.13
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false

[wshwsh12]

Duplicate of #23114
Only one issue is enough, I will fix it soon.

[ti-srebot]

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