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 82.92%. Comparing base (a882f34) to head (1485443).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #38813      +/-   ##
==========================================
- Coverage   82.93%   82.92%   -0.02%     
==========================================
  Files        1091     1091              
  Lines      169357   169393      +36     
==========================================
+ Hits       140453   140461       +8     
- Misses      23319    23333      +14     
- Partials     5585     5599      +14     

Components	Coverage Δ
Client	78.73% <ø> (ø)
Core	∅ <ø> (∅)
Go	83.07% <82.81%> (-0.02%)	⬇️

Files with missing lines	Coverage Δ
internal/querynodev2/segments/segment_loader.go	72.70% <100.00%> (-0.25%)	⬇️

... and 30 files with indirect coverage changes