ensure empty values are not deserialized #564

Job	Run time
find_package/linux/Release	4m 15s
find_package/linux/Debug	2m 54s
find_package/linux/libcxx/Release	6m 5s
find_package/linux/libcxx/Debug	4m 12s
find_package/macos/Release	9m 13s
find_package/macos/Debug	5m 43s
find_package/win/Release	6m 33s
find_package/win/Debug	3m 45s
find_library/linux/Release	4m 32s
find_library/linux/Debug	3m 10s
find_library/linux/libcxx/Release	5m 48s
find_library/linux/libcxx/Debug	6m 6s
find_library/macos/Release	13m 43s
find_library/macos/Debug	6m 14s
find_library/win/Release	5m 23s
find_library/win/Debug	4m 4s
singleheader/linux/Release	4m 27s
singleheader/linux/Debug	2m 42s
singleheader/linux/libcxx/Release	5m 25s
singleheader/linux/libcxx/Debug	3m 43s
singleheader/macos/Release	10m 10s
singleheader/macos/Debug	6m 4s
singleheader/win/Release	5m 20s
singleheader/win/Debug	4m 37s
	2h 14m 8s

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