executor,expression: avoid to append nil to warnings (#36304)

[ti-srebot]

cherry-pick #36304 to release-6.2
You can switch your code base to this Pull Request by using git-extras:

# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/36932

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/tidb.git pr/36932:release-6.2-6418350e9755

---

Signed-off-by: zyguan zhongyangguan@gmail.com

What problem does this PR solve?

Issue Number: close #31569

Problem Summary: two issues are described in #31569:

1. nil pointer exception was thrown when show warnings.
2. the warning messages were incompleted (missing template args).

What is changed and how it works?

Previously we always append the warning error even when it's a nil here, which leads to the first issue. This PR uses sc.HandleTruncate to handle trancate warnings uniformly and amends warnings generated on inserting.

Check List

Tests

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

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

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

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• tangenta
• xhebox

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[ti-srebot]

/run-all-tests

[ti-srebot]

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

[sre-bot]

Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/b1f9baf333948bd6f7570915c7f86f7e1daa75aa

[xhebox]

closed for DMR version