fix: [2.4] Fix incorrect memory estimation for small segments

[bigsheeper]

Skip estimation index memory logic for segments without index file.

issue: #37630

pr: #38813

[sre-ci-robot]

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[mergify]

@bigsheeper E2e jenkins job failed, comment /run-cpu-e2e can trigger the job again.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.40%. Comparing base (ccbe6fc) to head (8b0795a).

Additional details and impacted files

@@            Coverage Diff             @@
##              2.4   #38910      +/-   ##
==========================================
+ Coverage   80.48%   83.40%   +2.92%     
==========================================
  Files        1077      809     -268     
  Lines      169574   143079   -26495     
==========================================
- Hits       136483   119341   -17142     
+ Misses      28646    19267    -9379     
- Partials     4445     4471      +26     

Files with missing lines	Coverage Δ
internal/querynodev2/segments/segment_loader.go	71.41% <100.00%> (-0.23%)	⬇️

... and 306 files with indirect coverage changes