feat(python): Add Series/Expr.has_nulls and deprecate Series.has_validity

[stinodego]

The presence of a validity buffer is internals information that should not be part of the public API.

This PR adds a has_nulls method that can be used to check for null values. For now it's only available on the Python side, and the implementation is naive. We can optimize this later.

The has_nulls method should be added to the Rust side as well.

@orlp FYI - I'll leave the Rust side to you :)

[ritchie46]

Yes, I see merit in a has_nulls which we can later optimize. We don't have to trigger a full null count to answer that question.

[codecov]

Codecov Report

Attention: Patch coverage is 90.90909% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 81.48%. Comparing base (d68d499) to head (d19d650).
Report is 2 commits behind head on main.

Files	Patch %	Lines
py-polars/polars/series/series.py	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16488      +/-   ##
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+ Coverage   81.42%   81.48%   +0.05%     
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  Files        1409     1409              
  Lines      184924   184942      +18     
  Branches     2978     2981       +3     
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+ Hits       150577   150692     +115     
+ Misses      33826    33730      -96     
+ Partials      521      520       -1     

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[stinodego]

Yes, I see merit in a has_nulls which we can later optimize. We don't have to trigger a full null count to answer that question.

Shall I add a (naive) Series.has_nulls as part of this PR then, and deprecate in favor of that?

Other names I've considered are has_null, contains_nulls, or contains_null. Ultimately I think has_nulls is the best out of those 4.

[ritchie46]

Shall I add a (naive) Series.has_nulls as part of this PR then, and deprecate in favor of that?

Yes, let's do that. 👍