fix: [2.5] Fix incorrect memory estimation for small segments

[bigsheeper]

Skip estimation index memory logic for segments without index file.

issue: #37630

pr: #38813

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.45%. Comparing base (6fa096e) to head (89b237d).

❗ There is a different number of reports uploaded between BASE (6fa096e) and HEAD (89b237d). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (6fa096e)	HEAD (89b237d)
	2	1

Additional details and impacted files

@@             Coverage Diff             @@
##              2.5   #38909       +/-   ##
===========================================
- Coverage   81.05%   69.45%   -11.61%     
===========================================
  Files        1383      294     -1089     
  Lines      195503    26419   -169084     
===========================================
- Hits       158472    18349   -140123     
+ Misses      31457     8070    -23387     
+ Partials     5574        0     -5574     

Components	Coverage Δ
Client	∅ <ø> (∅)
Core	69.45% <ø> (-0.02%)	⬇️
Go	∅ <ø> (∅)

see 1090 files with indirect coverage changes