fix: flat FTS panic with prefilter

[BubbleCal]

No description provided.

[github-actions]

ACTION NEEDED
Lance follows the Conventional Commits specification for release automation.

The PR title and description are used as the merge commit message. Please update your PR title and description to match the specification.

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 78.73%. Comparing base (f508cbd) to head (f251280).

Files with missing lines	Patch %	Lines
rust/lance-index/src/scalar/inverted/index.rs	0.00%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3470      +/-   ##
==========================================
+ Coverage   78.67%   78.73%   +0.05%     
==========================================
  Files         251      251              
  Lines       92983    92983              
  Branches    92983    92983              
==========================================
+ Hits        73157    73206      +49     
+ Misses      16852    16798      -54     
- Partials     2974     2979       +5     

Flag	Coverage Δ
unittests	78.73% <0.00%> (+0.05%)	⬆️

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