refactor(rust): Ensure fallback node gets correct length df even if no columns selected

[orlp]

This fixed a bug for example in arg_sort_by where you select pl.int_range(0, pl.len()) which (currently) goes to the fallback, but doesn't actually select a column.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 7 lines in your changes missing coverage. Please review.

Project coverage is 79.84%. Comparing base (a53bf03) to head (45dac76).
Report is 38 commits behind head on main.

Files with missing lines	Patch %	Lines
...ates/polars-stream/src/physical_plan/lower_expr.rs	0.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18772      +/-   ##
==========================================
- Coverage   79.86%   79.84%   -0.02%     
==========================================
  Files        1517     1518       +1     
  Lines      205533   205576      +43     
  Branches     2892     2892              
==========================================
+ Hits       164142   164144       +2     
- Misses      40843    40884      +41     
  Partials      548      548              

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