fix: Incorrect height from slicing after projecting only the file path column

[nameexhaustion]

Fixes #20788

[codecov]

Codecov Report

Attention: Patch coverage is 76.00000% with 12 lines in your changes missing coverage. Please review.

Project coverage is 79.69%. Comparing base (b4b8f81) to head (fb25d27).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...polars-mem-engine/src/executors/multi_file_scan.rs	0.00%	12 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #20817   +/-   ##
=======================================
  Coverage   79.69%   79.69%           
=======================================
  Files        1568     1568           
  Lines      222969   222985   +16     
  Branches     2544     2544           
=======================================
+ Hits       177696   177717   +21     
+ Misses      44689    44684    -5     
  Partials      584      584           

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