executor: fix incorrect rows returned by TABLESAMPLE

[jingshanglu]

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

[CLAassistant]

All committers have signed the CLA.

[sre-bot]

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[ti-srebot]

No release note, Please follow https://github.com/pingcap/community/blob/master/contributors/release-note-checker.md

[sre-bot]

Please follow PR Title Format:

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[ti-srebot]

No release note, Please follow https://github.com/pingcap/community/blob/master/contributors/release-note-checker.md

[tangenta]

Could you please add a test in sample_test.go?

[jingshanglu]

Could you please add a test in sample_test.go?

ok

[sre-bot]

Please follow PR Title Format:

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[jingshanglu]

/run-check_title

[sre-bot]

Please follow PR Title Format:

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[jingshanglu]

/run-check_title

[bb7133]

LGTM

[ti-chi-bot]

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[tangenta]

/merge

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[tangenta]

/merge

[ti-chi-bot]

@tangenta: /merge is only allowed for the committers, you can assign this pull request to the committer in list by filling /assign @committer in the comment to help merge this pull request.

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/merge

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[tangenta]

/merge

[ti-chi-bot]

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

Commit hash: 604215a

[ti-srebot]

cherry pick to release-5.1 in PR #25430

[tangenta]

/run-cherry-picker

[ti-srebot]

cherry pick to release-5.0 in PR #25795