executor: fix unique index point get read with delete first (#16965)

[sre-bot]

cherry-pick #16965 to release-4.0

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What problem does this PR solve?

Problem Summary:

PointGet executor should process deleted index record with zero value length

mysql> create table uk(c1 int primary key, c2 int unique key);
Query OK, 0 rows affected (0.01 sec)

mysql> insert into uk values(1, 77);
Query OK, 1 row affected (0.00 sec)

mysql> begin;
Query OK, 0 rows affected (0.00 sec)

mysql> delete from uk where c1 = 1;
Query OK, 1 row affected (0.00 sec)

mysql> select * from uk where c2 = 77;
ERROR 1105 (HY000): runtime error: index out of range [7] with length 0

What is changed and how it works?

What's Changed:
Check index value got with zero-length

How it Works:

Related changes

• Need to cherry-pick to the release branch

Check List

Tests

• Unit test

Side effects

Release note

• Fix the panic issue that occurs when executing the PointGet query after the delete operation

[sre-bot]

/run-all-tests

[cfzjywxk]

/run-check_release_note

[jackysp]

/rebuild

[jackysp]

LGTM

[lzmhhh123]

LGTM

[jackysp]

/merge

[sre-bot]

/run-all-tests

[sre-bot]

@sre-bot merge failed.

[jackysp]

/rebuild

[lysu]

/rebuild