multiboot2: StringError::MissingNull => StringError::MissingNul #778

Job	Run time
build (msrv) / rust	31s
build (nightly) / rust	29s
build (stable) / rust	26s
build no_std (stable) [w/o builder] / rust	32s
build no_std (stable) / rust	37s
build no_std (stable) [Windows] / rust	1m 7s
style (stable) / rust	31s
build no_std (nightly) / rust	25s
style (nightly) / rust	42s
tests with miri (nightly) / rust	1m 7s
build no_std (msrv) / rust	28s
style (msrv) / rust	38s
	7m 33s

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