Fix NPE in S3PinotFS credential refresh in init

[deepthi912]

• S3PinotFS was built via the 3-arg init (e.g., by callers that supply their own pre-configured
  S3Client).
• The retry path calls initOrRefreshS3Client(), which immediately dereferences the null _s3Config

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.46%. Comparing base (dd6520c) to head (a8d62ed).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
.../org/apache/pinot/plugin/filesystem/S3PinotFS.java	0.00%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #18679      +/-   ##
============================================
- Coverage     64.47%   64.46%   -0.01%     
  Complexity     1291     1291              
============================================
  Files          3371     3371              
  Lines        208551   208551              
  Branches      32569    32569              
============================================
- Hits         134455   134438      -17     
- Misses        63292    63305      +13     
- Partials      10804    10808       +4     

Flag	Coverage Δ
custom-integration1	100.00% <ø> (ø)
integration	100.00% <ø> (ø)
integration1	100.00% <ø> (ø)
integration2	0.00% <ø> (ø)
java-21	64.46% <0.00%> (-0.01%)	⬇️
temurin	64.46% <0.00%> (-0.01%)	⬇️
unittests	64.46% <0.00%> (-0.01%)	⬇️
unittests1	56.89% <ø> (-0.02%)	⬇️
unittests2	37.09% <0.00%> (+<0.01%)	⬆️

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

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.