fix: handle filter on empty partition

[eddyxu]

It panicked when it applies filter on an empty partition

thread 'index::vector::pq::tests::test_filter_on_empty_pq_code' panicked at rust/lance/src/index/vector/pq.rs:125:14:
chunk size must be non-zero

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 48.38710% with 16 lines in your changes missing coverage. Please review.

Project coverage is 77.86%. Comparing base (8069936) to head (451d4c0).

Files with missing lines	Patch %	Lines
rust/lance/src/index/vector/pq.rs	48.38%	15 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3151      +/-   ##
==========================================
- Coverage   77.89%   77.86%   -0.04%     
==========================================
  Files         242      242              
  Lines       82183    82214      +31     
  Branches    82183    82214      +31     
==========================================
- Hits        64020    64016       -4     
- Misses      14973    14979       +6     
- Partials     3190     3219      +29     

Flag	Coverage Δ
unittests	77.86% <48.38%> (-0.04%)	⬇️

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