matroskadec: don't warn about unknown spherical medata when none is present

track->video.projection.type is set to 0 (a Matroska specific "No spherical
metadata present" value, with no related AVSphericalMapping) by default on
files without the element.

This removes bogus warnings on every single matroska file without Spherical
metadata.

Signed-off-by: James Almer <jamrial@gmail.com>