fix: Fix unpivot on empty df

[ritchie46]

fixes #18170

[codecov]

Codecov Report

Attention: Patch coverage is 95.51282% with 7 lines in your changes missing coverage. Please review.

Project coverage is 80.30%. Comparing base (21f02a6) to head (641c728).
Report is 4 commits behind head on main.

Files	Patch %	Lines
crates/polars-plan/src/plans/functions/schema.rs	20.00%	4 Missing ⚠️
crates/polars-plan/src/plans/functions/mod.rs	50.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18179      +/-   ##
==========================================
- Coverage   80.31%   80.30%   -0.02%     
==========================================
  Files        1498     1500       +2     
  Lines      198654   198669      +15     
  Branches     2831     2833       +2     
==========================================
- Hits       159555   159542      -13     
- Misses      38572    38600      +28     
  Partials      527      527              

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