fix: assert_is_empty clippy lint

[xtqqczze]

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

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/date/date-locale-hour (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/tail/pipe-f is now being skipped but was previously passing.

[xtqqczze]

CICD / Build (ubuntu-latest, x86_64-unknown-linux-gnu, feat_os_unix,test_risky_names, use-cross, true) (pull_request) failure tracked by #14251.