common: make it clear that the default values in Dav1dDataProps mean unknown

12 jobs for master in 7 minutes and 49 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #239277
amd64 debian
build-debian

00:00:32

passed #239281
debian aarch64
build-debian-aarch64

00:01:31

passed #239282
debian aarch64
build-debian-aarch64-clang-5

00:00:56

passed #239278
amd64 debian
build-debian-static

00:00:33

passed #239284
debian aarch64
build-debian-werror

00:00:31

passed #239283
macos
build-macos

00:00:26

passed #239279
win32
build-win32

00:00:31

passed #239280
win64
build-win64

00:00:36

 
  Test
passed #239285
amd64 debian
test-debian

00:00:48

passed #239286
amd64 debian
test-debian-asan

00:02:16

passed #239287
amd64 debian
test-debian-msan

00:01:04

passed #239288
amd64 debian
test-debian-ubsan

00:01:25