ddl: fix the optimization for the empty table(speed up)

[zimulala]

What problem does this PR solve?

Issue Number: close #49679, close #49682

Problem Summary:
The optimization for the empty table(startKey == nil && endKey == nil) doesn't work.
The following case is obvious (because split regions take so long).
#49679

[2023/12/20 11:23:10.873 +08:00] [INFO] [backfilling.go:712] ["start backfill workers to reorg record"] [category=ddl] [type="add index"] [workerCnt=4] [regionCnt=1] [startKey=] [endKey=]
...
[2023/12/20 11:25:14.752 +08:00] [INFO] [backfilling.go:559] ["get backfill range task, get reverse key failed"] [category=ddl] [error="[tikv:9005]Region is unavailable"]
...
...

What changed and how does it work?

Check List

Tests

• Unit test
• Integration test
  Before this PR, took time: 2min+
  

After this PR, took time: 40s+

• 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

[codecov]

Codecov Report

Merging #49629 (8f492a4) into master (210bc42) will increase coverage by 0.7322%.
Report is 82 commits behind head on master.
The diff coverage is 100.0000%.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #49629        +/-   ##
================================================
+ Coverage   71.0037%   71.7359%   +0.7322%     
================================================
  Files          1368       1429        +61     
  Lines        399344     430788     +31444     
================================================
+ Hits         283549     309030     +25481     
- Misses        96005     102862      +6857     
+ Partials      19790      18896       -894     

Flag	Coverage Δ
integration	45.0807% <100.0000%> (?)
unit	71.0034% <ø> (-0.0003%)	⬇️

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	46.3262% <ø> (-6.5668%)	⬇️

[tangenta]

Why not check this in origin place? Thus we can skip setupWorkers() and consumer.run()

[zimulala]

Because there will be return an error as described in #49682

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Defined2014, tangenta

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• pkg/ddl/OWNERS [tangenta]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-12-26 02:59:47.640626252 +0000 UTC m=+1534678.677853178: ☑️ agreed by tangenta.
• 2023-12-26 03:05:02.374525063 +0000 UTC m=+1534993.411751990: ☑️ agreed by Defined2014.

[wuhuizuo]

/retest

[tiprow]

@wuhuizuo: Cannot trigger testing until a trusted user reviews the PR and leaves an /ok-to-test message.

In response to this:

/retest

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #49814.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #49815.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #49816.

[ti-chi-bot]

/cherry-pick release-6.5

[ti-chi-bot]

In response to a cherrypick label: new pull request could not be created: failed to create pull request against pingcap/tidb#release-6.5 from head ti-chi-bot:cherry-pick-49629-to-release-6.5: status code 422 not one of [201], body: {"message":"Validation Failed","errors":[{"resource":"PullRequest","code":"custom","message":"A pull request already exists for ti-chi-bot:cherry-pick-49629-to-release-6.5."}],"documentation_url":"https://docs.github.com/rest/pulls/pulls#create-a-pull-request"}

[ti-chi-bot]

@ti-chi-bot: new pull request could not be created: failed to create pull request against pingcap/tidb#release-6.5 from head ti-chi-bot:cherry-pick-49629-to-release-6.5: status code 422 not one of [201], body: {"message":"Validation Failed","errors":[{"resource":"PullRequest","code":"custom","message":"A pull request already exists for ti-chi-bot:cherry-pick-49629-to-release-6.5."}],"documentation_url":"https://docs.github.com/rest/pulls/pulls#create-a-pull-request"}

In response to this:

/cherry-pick release-6.5

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