disttask: add sleep when subtask == nil

[ywqzzy]

What problem does this PR solve?

Issue Number: ref #46258

Problem Summary:

What is changed and how it works?

When subtask == nil but task is running, sleep for a while.
Very likely to happen when some tidb node is the tail, and some tidb node already processed all subtasks.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
  
  The cop request qps is really high.
• 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: tangenta, wjhuang2016

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-10-16 09:04:59.879001862 +0000 UTC m=+1647897.466112008: ☑️ agreed by tangenta.
• 2023-10-16 09:07:38.495513731 +0000 UTC m=+1648056.082623875: ☑️ agreed by wjhuang2016.

[codecov]

Codecov Report

Merging #47668 (0d44fde) into master (4b0b47c) will increase coverage by 0.9146%.
Report is 3 commits behind head on master.
The diff coverage is 100.0000%.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #47668        +/-   ##
================================================
+ Coverage   71.9282%   72.8428%   +0.9146%     
================================================
  Files          1396       1419        +23     
  Lines        404781     411152      +6371     
================================================
+ Hits         291152     299495      +8343     
+ Misses        94027      92826      -1201     
+ Partials      19602      18831       -771     

Flag	Coverage Δ
integration	41.1874% <0.0000%> (?)
unit	71.9476% <100.0000%> (+0.0193%)	⬆️

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

Components	Coverage Δ
dumpling	53.9913% <ø> (ø)
parser	∅ <ø> (∅)
br	48.6653% <ø> (-4.2648%)	⬇️