fix: preserve element indices schema for empty element-level search results

[SpadeA-Tang]

issue: #50010

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[sre-ci-robot]

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[SpadeA-Tang]

/set-milestone 3.0

[sre-ci-robot]

✅ CI Loop Results 003d86e

Stage	Result	Duration	Tests
✅ Build	SUCCESS	11.7min	-
✅ Code-Check	SUCCESS	7.3min	-
✅ UT-GO	SUCCESS	22.5min	1020 passed
✅ UT-Integration	SUCCESS	24.8min	46 passed
✅ UT-CPP-Cov	SUCCESS	43.2min	7838 passed

Total: 71min | Pipeline | Artifacts

Overall Coverage: 71.3%
Diff Coverage: CPP 97.7% (42 hit, 1 miss, 43 measurable lines, 15 unmeasured)
Diff Coverage HTML: view changed lines
Total Patch Coverage: 97.7% (42/43 measurable lines, 15 unmeasured)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.22%. Comparing base (fb65266) to head (003d86e).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #50417       +/-   ##
===========================================
+ Coverage   78.96%   85.22%    +6.25%     
===========================================
  Files        2239      671     -1568     
  Lines      396935   118435   -278500     
===========================================
- Hits       313430   100932   -212498     
+ Misses      73915    17451    -56464     
+ Partials     9590       52     -9538     

Components	Coverage Δ
Client	∅ <ø> (∅)
Core	85.95% <100.00%> (+<0.01%)	⬆️
Go	∅ <ø> (∅)

Files with missing lines	Coverage Δ
...ternal/core/src/exec/operator/VectorSearchNode.cpp	100.00% <100.00%> (ø)
internal/core/src/query/ExecPlanNodeVisitor.cpp	91.16% <100.00%> (+0.08%)	⬆️
internal/core/src/query/SearchOnGrowing.cpp	96.24% <100.00%> (+0.05%)	⬆️
internal/core/src/query/SearchOnSealed.cpp	97.14% <100.00%> (+0.08%)	⬆️
internal/core/src/query/Utils.h	80.35% <100.00%> (+0.35%)	⬆️
...ternal/core/src/segcore/search_result_export_c.cpp	70.72% <100.00%> (+0.76%)	⬆️
...l/core/src/segcore/search_result_export_c_test.cpp	98.59% <100.00%> (+0.03%)	⬆️

... and 1570 files with indirect coverage changes

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