header: fix authentication when protected header is zero-length map #269

Job	Run time
clippy	17s
build (stable)	24s
coverage	3m 23s
build (beta)	21s
deny	4m 40s
doc	18s
build (nightly-2023-04-01)	33s
examples (stable)	20s
formatting	55s
Rust 1.58.0 MSRV	2m 14s
examples (beta)	20s
Rust 1.59.0 MSRV	2m 13s
Build for a no_std target (stable)	18s
examples (nightly-2023-04-01)	23s
test (stable)	18s
Build for a no_std target (beta)	18s
test (beta)	17s
udeps	4m 17s
Build for a no_std target (nightly-2023-04-01)	32s
test (nightly-2023-04-01)	24s
	22m 45s

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