statistics: fix LoadStatsFromJSONNoUpdate func for CPUNum = 1

[Defined2014]

What problem does this PR solve?

Issue Number: close #49374

Problem Summary: When runtime.GOMAXPROCS(0) return 1, the concurrencyForPartition will be set to 0.

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

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[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:

• 2023-12-12 08:28:39.350765271 +0000 UTC m=+344810.387992199: ☑️ agreed by hawkingrei.
• 2023-12-12 08:31:37.287795008 +0000 UTC m=+344988.325021932: ☑️ agreed by hi-rustin.

[codecov]

Codecov Report

Merging #49375 (4ba93d0) into master (0b39f99) will increase coverage by 0.8371%.
Report is 3 commits behind head on master.
The diff coverage is 83.3333%.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #49375        +/-   ##
================================================
+ Coverage   71.0491%   71.8863%   +0.8371%     
================================================
  Files          1368       1411        +43     
  Lines        401712     414186     +12474     
================================================
+ Hits         285413     297743     +12330     
- Misses        96443      97637      +1194     
+ Partials      19856      18806      -1050     

Flag	Coverage Δ
integration	43.6174% <83.3333%> (?)
unit	71.0490% <ø> (-0.0001%)	⬇️

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

Components	Coverage Δ
dumpling	53.9663% <ø> (ø)
parser	∅ <ø> (∅)
br	47.9211% <ø> (-4.9723%)	⬇️