dumpling: force sort for table sample (#48305)

[ti-chi-bot]

This is an automated cherry-pick of #48305

What problem does this PR solve?

Issue Number: ref #48253

Problem Summary:

What is changed and how it works?

force sort for table sample by using COALESCE.

Check List

Tests

• Manual test (add detailed scripts or steps below)
  Manual using dumpling to dump false data and it works now.

Side effects

• None

Documentation

• None

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

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[ti-chi-bot]

@ti-chi-bot: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
idc-jenkins-ci-tidb/mysql-test	2e681e6	link	true	/test mysql-test

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