fix: Fix incorrect memory estimation for small segments

[bigsheeper]

Skip estimation index memory logic for segments without index file.

issue: #37630

[sre-ci-robot]

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[mergify]

@bigsheeper cpp-unit-test check failed, comment rerun cpp-unit-test can trigger the job again.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.93%. Comparing base (95c1ccc) to head (6a27227).
Report is 2 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #38813       +/-   ##
===========================================
+ Coverage   69.45%   82.93%   +13.48%     
===========================================
  Files         294     1091      +797     
  Lines       26430   169629   +143199     
===========================================
+ Hits        18357   140685   +122328     
- Misses       8073    23346    +15273     
- Partials        0     5598     +5598     

Components	Coverage Δ
Client	79.12% <ø> (∅)
Core	∅ <ø> (∅)
Go	83.08% <100.00%> (∅)

Files with missing lines	Coverage Δ
internal/querynodev2/segments/segment_loader.go	72.54% <100.00%> (ø)

... and 1384 files with indirect coverage changes

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