If the relevant elements (the color description elements for AV1 and the
VUI elements in general for H.264 (since 1156b507)) are absent, then their
correct values (usually meaning unknown) have already been inferred by
the reading process, so that it is unnecessary to initialize them again
in the av1/h264_metadata filters even when they were initially absent.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>