fix: filter the vec index in function Index::scalar_index_info

[SaintBacchus]

Create the vector index error when the schema has nullable field.

When the vector column had build index already, the dataset scaner will assign the filter in function compute_partitions.

    if dataset.schema.field(column).nullable and filter_nan:
        filt = f"{column} is not null"
    else:
        filt = None

The scanner will use the scalar index scanner and this scanner treat the vector index as scalar index(btree index).

This pr will filter the vector index in scalar_index_info and it will only return the scalar index now.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.80%. Comparing base (cdac5de) to head (c67edfd).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3000      +/-   ##
==========================================
+ Coverage   78.77%   78.80%   +0.02%     
==========================================
  Files         237      237              
  Lines       74099    74106       +7     
  Branches    74099    74106       +7     
==========================================
+ Hits        58374    58398      +24     
  Misses      12703    12703              
+ Partials     3022     3005      -17     

Flag	Coverage Δ
unittests	78.80% <100.00%> (+0.02%)	⬆️

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[wjones127]

Looks reasonable. Thank you! 👍