fix: Allow empty sort by columns

[orlp]

As I see it, sorting with by=[] is equivalent to sorting using the empty row as key value, and all empty rows are by definition equal. So this should just successfully return the original input. DataFrame.sort already did this, now Expr.sort_by also does.

[ritchie46]

I think you must update a test.

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 79.87%. Comparing base (6561eba) to head (3868fcf).
Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-core/src/frame/mod.rs	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18774      +/-   ##
==========================================
+ Coverage   79.84%   79.87%   +0.03%     
==========================================
  Files        1518     1518              
  Lines      205576   205615      +39     
  Branches     2892     2892              
==========================================
+ Hits       164132   164231      +99     
+ Misses      40896    40836      -60     
  Partials      548      548              

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