fix: Sortedness was incorrectly being preserved in dt.offset_by when offsetting by non-constant durations in the timezone-naive case

[MarcoGorelli]

closes #19608

[codecov]

Codecov Report

Attention: Patch coverage is 92.30769% with 1 line in your changes missing coverage. Please review.

Project coverage is 79.96%. Comparing base (0b0a914) to head (8108275).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-time/src/offset_by.rs	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #19616      +/-   ##
==========================================
- Coverage   79.96%   79.96%   -0.01%     
==========================================
  Files        1536     1536              
  Lines      211374   211374              
  Branches     2445     2445              
==========================================
- Hits       169034   169028       -6     
- Misses      41785    41791       +6     
  Partials      555      555              

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