refactor(rust): Fix zero-length len

[orlp]

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Please upload report for BASE (main@8114b52). Learn more about missing BASE report.
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-expr/src/reduce/convert.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #18817   +/-   ##
=======================================
  Coverage        ?   79.85%           
=======================================
  Files           ?     1518           
  Lines           ?   205637           
  Branches        ?     2893           
=======================================
  Hits            ?   164219           
  Misses          ?    40870           
  Partials        ?      548           

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