sessionctx: fix null max value to leading wrong warning (#57898)

[ti-chi-bot]

This is an automated cherry-pick of #57898

What problem does this PR solve?

Issue Number: close #57889

Problem Summary:

What changed and how does it work?

forget to set max value, it will judge wrongly when to verify value.

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.

fix null max value to leading wrong warning with tidb_gogc_tuner_max_value。tidb_gogc_tuner_min_value

修复 tidb_gogc_tuner_max_value/tidb_gogc_tuner_min_value 因为 空的 max value 导致错误的warning信息

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Please upload report for BASE (release-7.5@bf0766b). Learn more about missing BASE report.

Additional details and impacted files

@@               Coverage Diff                @@
##             release-7.5     #57933   +/-   ##
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  Coverage               ?   72.5705%           
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  Files                  ?       1415           
  Lines                  ?     415302           
  Branches               ?          0           
================================================
  Hits                   ?     301387           
  Misses                 ?      94094           
  Partials               ?      19821           

Flag	Coverage Δ
unit	72.5705% <100.0000%> (?)

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

Components	Coverage Δ
dumpling	52.9400% <0.0000%> (?)
parser	∅ <0.0000%> (?)
br	53.5565% <0.0000%> (?)

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hawkingrei, yudongusa

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Needs approval from an approver in each of these files:

• OWNERS [hawkingrei]
• pkg/sessionctx/variable/OWNERS [yudongusa]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-12-04 06:25:43.868485503 +0000 UTC m=+1222531.488140016: ☑️ agreed by hawkingrei.
• 2024-12-04 06:30:46.993188259 +0000 UTC m=+1222834.612842775: ☑️ agreed by yudongusa.