executor: fix incorrect rows returned by TABLESAMPLE (#25357)

[ti-srebot]

cherry-pick #25357 to release-5.0
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git pr https://github.com/pingcap/tidb/pull/25795

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---

What problem does this PR solve?

Issue Number: close #25257
Problem Summary:
Startup parameter --split-table=false, TableSampleExecutor does not handle the case where the end of the region is nil.

Tests

• Unit test
• Manual test

create table test.tsample1(id int primary key);
create table test.tsample2(id int primary key);
insert into test.tsample2 values(1);
select * from test.tsample1 tablesample regions();

Release note

• Add the processing of region end as nil

[ti-srebot]

/run-all-tests

[ti-srebot]

@jingshanglu you're already a collaborator in bot's repo.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• AilinKid
• bb7133

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[bb7133]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: fa28ae2

[ti-chi-bot]

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[zhouqiang-cl]

/run-unit-test