ln: don't panic on --relative with a parentless target

[leeewee]

Fixes #12628

ln -rsfT SRC / (and … "") aborted with an Option::unwrap() panic before any filesystem operation: relative_path computed the destination's parent with dst.parent().unwrap(), but Path::parent() is None for a parentless destination (/, "", or a bare Windows prefix).

Fold dst.parent() into the guard and fall through to the non-relative src when it's None, so the caller reports the usual error (exit 1) instead of crashing. Adds a regression test.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/tail-n0f (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/tail/pipe-f is now passing!
Skip an intermittent issue tests/pr/bounded-memory (was skipped on 'main', now failing)

[codspeed-hq]

Merging this PR will not alter performance

✅ 323 untouched benchmarks
⏩ 46 skipped benchmarks¹

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_{Comparing leeewee:ln-fix-relative-parentless-target (0bae620) with main (abcdcdc)}

Footnotes

1. 46 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩