fix: Wrap with fboundp for directory-empty-p #733

Job	Run time
test (ubuntu-latest, 26.3, false)	1m 9s
test (ubuntu-latest, 27.2, false)	1m 8s
test (ubuntu-latest, 28.2, false)	1m 13s
test (ubuntu-latest, 29.1, false)	1m 14s
test (macos-latest, 26.3, false)	3m 33s
test (macos-latest, 27.2, false)	3m 13s
test (macos-latest, 28.2, false)	5m 3s
test (macos-latest, 29.1, false)	2m 49s
test (windows-latest, 26.3, false)	4m 28s
test (windows-latest, 27.2, false)	2m 1s
test (windows-latest, 28.2, false)	2m 11s
test (windows-latest, 29.1, false)	1m 53s
test (ubuntu-latest, snapshot, true)	1m 12s
test (macos-latest, snapshot, true)	2m 1s
test (windows-latest, snapshot, true)	58s
	34m 6s

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