expression: set a collation according to the arguments for ifnull in constant folding (#52119)

[ti-chi-bot]

This is an automated cherry-pick of #52119

What problem does this PR solve?

Issue Number: close #51765

Problem Summary:

The ifnull has different behavior in inferring collation for constant folding and normal execution path.

What changed and how does it work?

Set the collation according to the first argument in the path of constant folding.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

Release note

Fix the issue that the collation of `ifnull` expression is different with MySQL when constant folding applies.

[YangKeao]

LGTM

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (release-7.1@ca6a857). Click here to learn what that means.
The diff coverage is 100.0000%.

Additional details and impacted files

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: YangKeao, zimulala

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• OWNERS [YangKeao,zimulala]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-04-11 08:07:14.011923178 +0000 UTC m=+1122495.539463723: ☑️ agreed by YangKeao.
• 2024-04-12 08:02:35.497474377 +0000 UTC m=+1208617.025014922: ☑️ agreed by zimulala.