xargs: do not panic on empty input in replace mode

[leeewee]

Fixes #735

xargs -I {} (and -i, --replace) panicked on empty input:

$ printf '' | xargs -I {} echo hello {}
thread 'main' panicked at src/xargs/mod.rs:429:46:
index out of bounds: the len is 0 but the index is 0
$ echo $?
101

In replace mode xargs runs the command once per input item, so with empty input it should run nothing. Instead the final builder was still executed and CommandBuilder::execute indexed extra_args[0], which is empty when no items were read.

This treats replace mode as implying --no-run-if-empty (as GNU does), so empty input runs nothing and exits 0:

$ printf '' | xargs -I {} echo hello {} ; echo $?
0

Non-empty replace input still runs per item, and non-replace / -r behavior is unchanged. Added a regression test (xargs_replace_empty_input).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.87%. Comparing base (795d29a) to head (02133dc).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #736      +/-   ##
==========================================
+ Coverage   91.86%   91.87%   +0.01%     
==========================================
  Files          35       35              
  Lines        7153     7152       -1     
  Branches      375      376       +1     
==========================================
  Hits         6571     6571              
  Misses        437      437              
+ Partials      145      144       -1     

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[codspeed-hq]

Merging this PR will not alter performance

✅ 20 untouched benchmarks

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_{Comparing leeewee:xargs-fix-replace-empty-input (02133dc) with main (795d29a)}