[MINOR] Fix some bad practices reported by spotbugs

[kaijchen]

What changes were proposed in this pull request?

Fix some bad practices reported by spotbugs (medium priority).

Why are the changes needed?

To improve code quality.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

CI with extra spotbugs rules: https://github.com/kaijchen/incubator-uniffle/actions/runs/2902861660

[codecov-commenter]

Codecov Report

Merging #177 (578f100) into master (df4163f) will decrease coverage by 0.11%.
The diff coverage is 40.00%.

@@             Coverage Diff              @@
##             master     #177      +/-   ##
============================================
- Coverage     58.29%   58.18%   -0.12%     
  Complexity     1263     1263              
============================================
  Files           158      158              
  Lines          8400     8408       +8     
  Branches        780      782       +2     
============================================
- Hits           4897     4892       -5     
- Misses         3251     3262      +11     
- Partials        252      254       +2     

Impacted Files	Coverage Δ
...org/apache/uniffle/common/config/ConfigOption.java	60.86% <0.00%> (ø)
...java/org/apache/uniffle/common/config/RssConf.java	28.47% <ø> (-0.50%)	⬇️
...org/apache/uniffle/server/LocalStorageChecker.java	67.74% <0.00%> (-1.83%)	⬇️
...org/apache/uniffle/server/ShuffleFlushManager.java	78.33% <0.00%> (-1.00%)	⬇️
...rg/apache/uniffle/storage/common/LocalStorage.java	43.66% <0.00%> (-1.02%)	⬇️
.../org/apache/uniffle/common/util/UnitConverter.java	47.36% <33.33%> (+0.22%)	⬆️
...pache/spark/shuffle/writer/WriteBufferManager.java	83.45% <100.00%> (ø)
...oordinator/PartitionBalanceAssignmentStrategy.java	98.46% <100.00%> (ø)
...he/uniffle/server/buffer/ShuffleBufferManager.java	82.68% <100.00%> (+0.43%)	⬆️
...pache/uniffle/storage/common/LocalStorageMeta.java	75.72% <100.00%> (ø)
... and 2 more

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[jerqi]

LGTM, thanks @kaijchen