planner: set right null flag for constant value

[fzhedu]

What problem does this PR solve?

Issue Number: close #xxx

Problem Summary: #14912 did not thoroughly set right null flag for constant value, so this PR aims to do that. It has to keep the expression and schema item consistent. For example: select -1 from t; the expression (-1) is not null, and its returning schema contains a not-null column. As a result, plan_to_pb will get consistent plan, avoiding errors in TiFlash.

What is changed and how it works?

Proposal: xxx

What's Changed:

we set null flag for constants in two aspects:

1. a constant item;
2. folding constants.

How it Works:

Related changes

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

Check List

Tests

• Integration test (the existing typeinfer_test cover the goal, which checks the schema and expression.)

Release note

• No release note

[fzhedu]

/cc @lzmhhh123 @leiysky @windtalker

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• hanfei1991
• lzmhhh123

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[hanfei1991]

/merge

[ti-chi-bot]

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

Commit hash: d8a1eb9

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