port: Allow creating an empty set. #287

Job	Run time
miri	1m 36s
no_std	30s
build (ubuntu, stable, locked, true)	12m 13s
build (windows, stable, locked, false)	21m 43s
build (macos, stable, locked, false)	18m 8s
build (ubuntu, nightly, locked, false)	20m 3s
build (ubuntu, beta, locked, false)	12m 15s
build (ubuntu, stable, latest, false)	16m 6s
build (ubuntu, stable, minimal, false)	15m 16s
deploy	5s
fuzz	15m 12s
features	25m 23s
	2h 38m 30s

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