Fix that coalesce mistakenly removed nullable from the result column (#3394)

[ti-chi-bot]

This is an automated cherry-pick of #3394

What problem does this PR solve?

Issue Number: close #3388

Problem Summary:

Coalesce will align the nullable property of result column with the last argument column. However Const(Nullable) is not treated as nullable and if the last argument column is Const(Nullable) coalesce will return String instead of Nullalbe(String).

What is changed and how it works?

Proposal: xxx

What's Changed:

Align the nullable property of result column with the last argument datatype.

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• Need to cherry-pick to the release branch:

Check List

Tests

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

Side effects

Release note

Fix that coalesce mistakenly removed nullable flag from the result column.

[ti-chi-bot]

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• windtalker

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[windtalker]

/rebuild

[windtalker]

LGTM

[windtalker]

/merge

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[ti-chi-bot]

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

Commit hash: ec7f9fd