ensure empty values are not deserialized #565

Job	Run time
find_package/linux/Release	4m 17s
find_package/linux/Debug	2m 38s
find_package/linux/libcxx/Release	6m 22s
find_package/linux/libcxx/Debug	3m 37s
find_package/macos/Release	7m 32s
find_package/macos/Debug	5m 42s
find_package/win/Release	5m 33s
find_package/win/Debug	3m 49s
find_library/linux/Release	4m 15s
find_library/linux/Debug	2m 55s
find_library/linux/libcxx/Release	5m 45s
find_library/linux/libcxx/Debug	3m 35s
find_library/macos/Release	7m 48s
find_library/macos/Debug	5m 55s
find_library/win/Release	4m 51s
find_library/win/Debug	3m 42s
singleheader/linux/Release	4m 15s
singleheader/linux/Debug	2m 48s
singleheader/linux/libcxx/Release	5m 23s
singleheader/linux/libcxx/Debug	4m 33s
singleheader/macos/Release	7m 39s
singleheader/macos/Debug	6m 30s
singleheader/win/Release	5m 15s
singleheader/win/Debug	4m 51s
	1h 59m 30s

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