planner: generate correct number of rows when all agg funcs are pruned (#24937)

[ti-srebot]

cherry-pick #24937 to release-5.0
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# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/25062

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---

What problem does this PR solve?

Issue Number: close #24865

Problem Summary:

Wrong query result.

What is changed and how it works?

What's Changed:
If all aggregation funcs are pruned during logical optimization, keep a count(1) instead of first_row(1) if the original LogicalAggregation contains agg funcs other than first_row().

How it Works:
See the code comments for explanations.

Related changes

• Need to cherry-pick to the release branch

Check List

Tests

• Unit test

Side effects

N/A

Release note

• Generate correct number of rows when all agg funcs are pruned

[ti-srebot]

/run-all-tests

[ti-srebot]

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

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• qw4990
• winoros

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[qw4990]

/merge

[ti-chi-bot]

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

Commit hash: d9e3af0

[ti-chi-bot]

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