fix: don't always include first doc

[wjones127]

Fixes #2930

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 50.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 78.99%. Comparing base (396b4d1) to head (cd16d12).
Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
rust/lance-index/src/scalar/inverted/wand.rs	40.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2957   +/-   ##
=======================================
  Coverage   78.99%   78.99%           
=======================================
  Files         234      234           
  Lines       72829    72833    +4     
  Branches    72829    72833    +4     
=======================================
+ Hits        57530    57535    +5     
+ Misses      12329    12328    -1     
  Partials     2970     2970           

Flag	Coverage Δ
unittests	78.99% <50.00%> (+<0.01%)	⬆️

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[westonpace]

Good work! Thanks for fixing :)