ddl: fix the optimization for the empty table(speed up) (#49629)

[ti-chi-bot]

This is an automated cherry-pick of #49629

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

❗ No coverage uploaded for pull request base (release-7.5@20c3e7d). Click here to learn what that means.
The diff coverage is 100.0000%.

Additional details and impacted files

@@               Coverage Diff                @@
##             release-7.5     #49816   +/-   ##
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Flag	Coverage Δ
unit	72.7330% <100.0000%> (?)

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

Components	Coverage Δ
dumpling	53.9913% <0.0000%> (?)
parser	∅ <0.0000%> (?)
br	53.6105% <0.0000%> (?)

[zimulala]

/retest-required

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-02-18 08:51:25.665916867 +0000 UTC m=+174374.413539978: ☑️ agreed by Defined2014.
• 2024-02-18 08:57:09.044338641 +0000 UTC m=+174717.791961790: ☑️ agreed by tangenta.