fix: flat FTS would return all unindexed rows

[BubbleCal]

No description provided.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 88.34951% with 12 lines in your changes missing coverage. Please review.

Project coverage is 78.49%. Comparing base (b572905) to head (abb5950).

Files with missing lines	Patch %	Lines
rust/lance/src/index.rs	89.24%	0 Missing and 10 partials ⚠️
rust/lance-index/src/scalar/inverted/index.rs	80.00%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #3386    +/-   ##
========================================
  Coverage   78.49%   78.49%            
========================================
  Files         250      250            
  Lines       90243    90343   +100     
  Branches    90243    90343   +100     
========================================
+ Hits        70833    70913    +80     
- Misses      16505    16514     +9     
- Partials     2905     2916    +11     

Flag	Coverage Δ
unittests	78.49% <88.34%> (+<0.01%)	⬆️

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