fix: Only slice after sort when slice is smaller than frame length

[mcrumiller]

Closes #20079

>>> import polars as pl
>>> pl.LazyFrame({"a": [None, 1.2]}).sort("a", nulls_last=True).slice(0, 10).collect()
shape: (2, 1)
┌──────┐
│ a    │
│ ---  │
│ f64  │
╞══════╡
│ 1.2  │
│ null │
└──────┘

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.53%. Comparing base (954000c) to head (1d558c2).
Report is 9 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #20084   +/-   ##
=======================================
  Coverage   79.52%   79.53%           
=======================================
  Files        1563     1563           
  Lines      217124   217124           
  Branches     2464     2464           
=======================================
+ Hits       172674   172689   +15     
+ Misses      43890    43875   -15     
  Partials      560      560           

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[ritchie46]

Cheers.