Produce an error if the user specifies an empty conditional inclusion… #123

Job	Run time
msrv	2m 46s
Check clippy	25s
Run tests	30s
Check format	11s
Publish	38s
release artifact for x86_64-pc-windows-gnu	1m 18s
release artifact for x86_64-pc-windows-msvc	1m 23s
release artifact for x86_64-apple-darwin	55s
release artifact for aarch64-apple-darwin	1m 6s
release artifact for x86_64-unknown-linux-musl	40s
release artifact for x86_64-unknown-linux-gnu	37s
	10m 29s

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