statistics: fix wrong NDV in the global stats when to disable async-merge-global-stats (#53762)

[ti-chi-bot]

This is an automated cherry-pick of #53762

What problem does this PR solve?

Issue Number: close #53752

Problem Summary:

What changed and how does it work?

blockMergeGlobalStats doesn't notice that FMSketch contains empty objectives in all partition's stats. so we need to find nil to init global rightly.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

run as the PR's description.

• No need to test

  • I checked and no code files have been changed.

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]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hawkingrei, hi-rustin

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-06-04 11:56:32.667552745 +0000 UTC m=+3382346.424688310: ☑️ agreed by hawkingrei.
• 2024-06-05 02:07:56.143614129 +0000 UTC m=+3433429.900749686: ☑️ agreed by hi-rustin.