clippy: fix assert_is_empty lint

[xtqqczze]

https://rust-lang.github.io/rust-clippy/main/index.html#assert_is_empty

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/io-errors (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/symlink (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/rm/isatty (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/tail-n0f (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/printf/printf-surprise is now being skipped but was previously passing.

[codspeed-hq]

Merging this PR will improve performance by 6.12%

⚡ 1 improved benchmark
✅ 360 untouched benchmarks
⏩ 50 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	ls_recursive_deep_tree[(200, 2)]	3.4 ms	3.2 ms	+6.12%

Tip

Curious why performance improved? Comment @codspeedbot explain why performance improved on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing xtqqczze:clippy/assert_is_empty (43b30ca) with main (883b625)²}

Footnotes

1. 50 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. ↩

2. No successful run was found on main (ddc98c2) during the generation of this report, so 883b625 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩