restore: fix choose the emptry key as split key (#50478)

[ti-chi-bot]

This is an automated cherry-pick of #50478

What problem does this PR solve?

Issue Number: close #50471 #50477

Problem Summary:

1. new split approach will choose empty key in some case. this PR tried to fix it.

What changed and how does it work?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• 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: BornChanger, Leavrth

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• br/OWNERS [BornChanger,Leavrth]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-01-17 12:28:26.072112738 +0000 UTC m=+358947.636410443: ☑️ agreed by BornChanger.
• 2024-01-17 12:39:21.938566718 +0000 UTC m=+359603.502864422: ☑️ agreed by Leavrth.

[BornChanger]

/retest

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (release-7.6@6c576f9). Click here to learn what that means.
The diff coverage is n/a.

Additional details and impacted files

@@               Coverage Diff                @@
##             release-7.6     #50518   +/-   ##
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  Coverage               ?   70.0209%           
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  Files                  ?       1436           
  Lines                  ?     419263           
  Branches               ?          0           
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  Hits                   ?     293572           
  Misses                 ?     105389           
  Partials               ?      20302           

Flag	Coverage Δ
unit	70.0209% <0.0000%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
dumpling	54.0269% <0.0000%> (?)
parser	∅ <0.0000%> (?)
br	49.6778% <0.0000%> (?)

[BornChanger]

/retest