fix: Don't access out-of-bounds for null indices in bitmap gather

[orlp]

Fixes #14927.

[ritchie46]

Can you add the repro as test?

[codecov]

Codecov Report

Attention: Patch coverage is 47.61905% with 11 lines in your changes are missing coverage. Please review.

Project coverage is 81.01%. Comparing base (d9dead6) to head (a4c95f8).
Report is 6 commits behind head on main.

Files	Patch %	Lines
crates/polars-arrow/src/compute/take/bitmap.rs	47.36%	10 Missing ⚠️
crates/polars-arrow/src/compute/take/boolean.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14932      +/-   ##
==========================================
- Coverage   81.01%   81.01%   -0.01%     
==========================================
  Files        1332     1332              
  Lines      172870   172882      +12     
  Branches     2458     2458              
==========================================
+ Hits       140054   140063       +9     
- Misses      32348    32351       +3     
  Partials      468      468              

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