statistics: fix merge global stats panic if the partition hasn't been analyzed before

[Yisaer]

Signed-off-by: yisaer disxiaofei@163.com

What problem does this PR solve?

Issue Number: close #39121

Problem Summary:

What is changed and how it works?

fix merge global stats panic if the partition hasn't been analyzed before

Check List

Tests

• Unit test

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:

• chrysan
• xhebox

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[xhebox]

/merge

[ti-chi-bot]

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

Commit hash: d35383e

[sre-bot]

TiDB MergeCI notify

✅ Well Done! New fixed [3] after this pr merged.

CI Name	Result	Duration	Compare with Parent commit
idc-jenkins-ci-tidb/integration-ddl-test	🔴 failed 1, success 5, total 6	44 min	Existing failure
idc-jenkins-ci/integration-cdc-test	🔴 failed 1, success 38, total 39	19 min	Existing failure
idc-jenkins-ci-tidb/common-test	✅ all 11 tests passed	17 min	Fixed
idc-jenkins-ci-tidb/integration-common-test	✅ all 17 tests passed	13 min	Fixed
idc-jenkins-ci-tidb/mybatis-test	✅ all 1 tests passed	5 min 3 sec	Fixed
idc-jenkins-ci-tidb/tics-test	🟢 all 1 tests passed	6 min 3 sec	Existing passed
idc-jenkins-ci-tidb/sqllogic-test-1	🟢 all 26 tests passed	5 min 34 sec	Existing passed
idc-jenkins-ci-tidb/sqllogic-test-2	🟢 all 28 tests passed	4 min 43 sec	Existing passed
idc-jenkins-ci-tidb/integration-compatibility-test	🟢 all 1 tests passed	2 min 54 sec	Existing passed
idc-jenkins-ci-tidb/plugin-test	🟢 build success, plugin test success	4min	Existing passed