fix: handle null vectors in flat search

[wjones127]

No description provided.

[wjones127]

This change was needed to get the existing test to pass in CI. It does seem suspicious though: why would we get an empty code book in PQ if we have nulls?

[wjones127]

I think it would be fine to merge this for now, to fix the immediate bug, but to come back to this an investigate later.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 98.59155% with 1 line in your changes missing coverage. Please review.

Project coverage is 78.89%. Comparing base (bfacd7c) to head (98e0f88).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
rust/lance-index/src/vector/pq/distance.rs	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3422      +/-   ##
==========================================
+ Coverage   78.85%   78.89%   +0.03%     
==========================================
  Files         250      250              
  Lines       91474    91602     +128     
  Branches    91474    91602     +128     
==========================================
+ Hits        72134    72265     +131     
+ Misses      16379    16375       -4     
- Partials     2961     2962       +1     

Flag	Coverage Δ
unittests	78.89% <98.59%> (+0.03%)	⬆️

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