xargs: treat whitespace-only input as no argument (fixes #771)

[MsfPablo]

Fixes #771.

Problem

When a filename (or any input) given to xargs consists solely of an ASCII space, uu xargs passes an empty-string argument to the child process:

$ echo -n " " > a.txt
$ ./target/debug/xargs -a a.txt ls
ls: cannot access ''

GNU xargs instead treats the lone space as a delimiter and runs the child with no extra arguments:

$ xargs -a a.txt ls
<lists the cwd, exit 0>

The same discrepancy occurs for any whitespace-only input (space, tab, newline, runs of them) on both stdin and -a.

Root cause

In src/xargs/mod.rs, the WhitespaceDelimitedArgumentReader::next loop returns Ok(None) at EOF only when no bytes have been read at all (i == 0). Input that was consumed but consisted purely of delimiters left result empty while i > 0, so the loop fell through and emitted a zero-length Argument. xargs then passed that empty string to the child.

Fix

At EOF, return Ok(None) whenever result is empty, not only when i == 0. This makes whitespace-only input behave exactly like empty input (delimiters separate arguments; a run of delimiters with no content between them yields no argument), matching GNU xargs. The i == 0 case is subsumed since no bytes read implies result is empty.

Behavior of normal tokenization (leading/trailing/multiple delimiters around real content, quoted/escaped args) is unchanged.

Test

Added xargs_whitespace_only_input covering space, tab, mixed whitespace, --no-run-if-empty, and the -a repro from the issue (asserting the child receives zero xargs-provided arguments). All existing xargs tests still pass.

cargo test          # 226 + 28 (xargs) tests, all pass
cargo fmt --check   # clean
cargo clippy --all-targets -- -D warnings   # clean

The acceptance criterion is GNU xargs behavior: whitespace-only input yields no argument.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.93%. Comparing base (1f19cdd) to head (46778db).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #819   +/-   ##
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  Coverage   91.93%   91.93%           
=======================================
  Files          35       35           
  Lines        7251     7251           
  Branches      378      378           
=======================================
  Hits         6666     6666           
  Misses        443      443           
  Partials      142      142           

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[github-actions]

Commit 46778db has test result changes:

GNU findutils testsuite:

Test results comparison:
  Current:   TOTAL: 495 / PASSED: 418 / FAILED: 76 / SKIPPED: 1
  Reference: TOTAL: 495 / PASSED: 417 / FAILED: 77 / SKIPPED: 1

Changes from main branch:
  TOTAL: +0
  PASSED: +1
  FAILED: -1

Test improvements (1):
  + r

bfs testsuite:

Test results comparison:
  Current:   TOTAL: 314 / PASSED: 267 / FAILED: 41 / SKIPPED: 6
  Reference: TOTAL: 312 / PASSED: 266 / FAILED: 40 / SKIPPED: 6

Changes from main branch:
  TOTAL: +2
  PASSED: +1
  FAILED: +1

New test failures (1):
  - gnu/okdir_path_relative

[codspeed-hq]

Merging this PR will not alter performance

✅ 20 untouched benchmarks

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_{Comparing MsfPablo:fix/xargs-space-only-input-771 (46778db) with main (1f19cdd)}