staistics: fix load stats from old version json | tidb-test=pr/2114 (#42967)

[ti-chi-bot]

This is an automated cherry-pick of #42967

What problem does this PR solve?

Issue Number: close #42931

Problem Summary:

What is changed and how it works?

When v4.0.x and earlier versions dump stats json, stats json doesn't contain StatsVer. Before the PR, when we find stats json does not contain StatsVer, we just simply set its StatsVer to 1. Another fact is that we use StatsVer != 0 to decide whether stats are collected(i.e., analyze has been executed). Setting StatsVer to 1 for stats json from old versions makes us unable to distinguish whether stats are collected or not. Hence, after the PR, when we find stats json does not contain StatsVer, we check NDV > 0 || NullCount > 0 to decide whether stats are collected. If stats are collected, we set its StatsVer to 1, otherwise we set its StatsVer to 0.

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]

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[ti-chi-bot]

@ti-chi-bot: This cherry pick PR is for a release branch and has not yet been approved by release team.
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[ti-chi-bot]

@ti-chi-bot: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
idc-jenkins-ci-tidb/mysql-test	029722d	link	true	/test mysql-test
idc-jenkins-ci-tidb/build	029722d	link	true	/test build
idc-jenkins-ci-tidb/check_dev	029722d	link	true	/test check-dev
idc-jenkins-ci-tidb/check_dev_2	029722d	link	true	/test check-dev2
idc-jenkins-ci-tidb/unit-test	029722d	link	true	/test unit-test

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