xargs: don't emit an empty argument for a trailing blank

[kevinburke]

When input ended with a blank immediately before its final newline, xargs passed one extra empty argument to the command:

$ printf 'aaa \nbbb \n' | xargs printf '[%s]'
[aaa][bbb][]        # was
[aaa][bbb]          # GNU findutils 4.10.0, and now

WhitespaceDelimitedArgumentReader::next used one result.is_empty() check to answer two different questions. The whitespace branch broke out of the loop only if result was non-empty, so a delimiter seen before any token was skipped -- correct for blanks, but it left "we are mid-token" indistinguishable from "we have not started one". The EOF branch then used i == 0 as a proxy for "nothing was consumed", which is false for a final call that consumes only the trailing newline, so it flushed a zero-length token. Input ending without a newline happened to work because nothing was left to consume.

Track that state explicitly with an in_argument flag, mirroring seen_arg in GNU's read_line, and test result.is_empty() at EOF. A delimiter now ends an argument only when we are inside one, and EOF flushes only a non-empty buffer.

This also fixes a second symptom of the same conflation: quoted empty arguments were dropped mid-stream, since quotes start an argument without contributing any bytes. printf '"" x\n' gave [x] and now gives [][x], again matching GNU. An unterminated x '' at end of input still yields just [x], as GNU's if (p == linebuf) return -1 does.

-I/-i is unaffected: with a replace string and no explicit delimiter, xargs uses the newline-delimited reader instead.

[codspeed-hq]

Merging this PR will not alter performance

✅ 20 untouched benchmarks

---

_{Comparing kevinburke:xargs-trailing-blank (bd7e3fe) with main (58c3af0)}

[github-actions]

Commit bd7e3fe has test result changes:

GNU findutils testsuite:

Test results comparison:
  Current:   TOTAL: 495 / PASSED: 419 / FAILED: 75 / SKIPPED: 1
  Reference: TOTAL: 495 / PASSED: 417 / FAILED: 77 / SKIPPED: 1

Changes from main branch:
  TOTAL: +0
  PASSED: +2
  FAILED: -2

Test improvements (2):
  + r
  + sv-bug-18713

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.99%. Comparing base (58c3af0) to head (bd7e3fe).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #829      +/-   ##
==========================================
+ Coverage   91.93%   91.99%   +0.05%     
==========================================
  Files          35       35              
  Lines        7253     7304      +51     
  Branches      378      378              
==========================================
+ Hits         6668     6719      +51     
  Misses        443      443              
  Partials      142      142              

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