ensure empty values are not deserialized #567

Job	Run time
find_package/linux/Release	3m 54s
find_package/linux/Debug	2m 32s
find_package/linux/libcxx/Release	3m 53s
find_package/linux/libcxx/Debug	2m 48s
find_package/macos/Release	13m 42s
find_package/macos/Debug	5m 53s
find_package/win/Release	5m 8s
find_package/win/Debug	3m 31s
find_library/linux/Release	4m 1s
find_library/linux/Debug	2m 21s
find_library/linux/libcxx/Release	6m 16s
find_library/linux/libcxx/Debug	3m 48s
find_library/macos/Release	7m 18s
find_library/macos/Debug	5m 52s
find_library/win/Release	5m 7s
find_library/win/Debug	3m 55s
singleheader/linux/Release	4m 9s
singleheader/linux/Debug	2m 50s
singleheader/linux/libcxx/Release	4m 26s
singleheader/linux/libcxx/Debug	2m 54s
singleheader/macos/Release	7m 38s
singleheader/macos/Debug	5m 55s
singleheader/win/Release	5m 12s
singleheader/win/Debug	3m 48s
	1h 56m 51s

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