br: fix unstable test br_300_small_tables (#46560)

[ti-chi-bot]

This is an automated cherry-pick of #46560

What problem does this PR solve?

Issue Number: close #46561

Problem Summary: Sometimes, when the backupsize is exactly an integer, it can cause the test script to receive a null value and unexpectedly exit.

What is changed and how it works?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

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-6.5@d7ce2f2). Click here to learn what that means.
The diff coverage is n/a.

Additional details and impacted files

@@               Coverage Diff                @@
##             release-6.5     #46567   +/-   ##
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  Coverage               ?   73.8184%           
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  Files                  ?       1085           
  Lines                  ?     350541           
  Branches               ?          0           
================================================
  Hits                   ?     258764           
  Misses                 ?      75295           
  Partials               ?      16482           

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[purelind]

/test pull-br-integration-test

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@purelind: No presubmit jobs available for pingcap/tidb@release-6.5

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/test pull-br-integration-test

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[purelind]

/test pull-br-integration-test

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@purelind: No presubmit jobs available for pingcap/tidb@release-6.5

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/test pull-br-integration-test

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[ti-chi-bot]

@ti-chi-bot: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-br-integration-test	374d347	link	false	/test pull-br-integration-test

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: 3pointer, BornChanger

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-09-07 11:21:33.696939665 +0000 UTC m=+2617258.245955652: ☑️ agreed by BornChanger.
• 2023-09-08 06:09:39.520079728 +0000 UTC m=+2684944.069095715: ☑️ agreed by 3pointer.