Remove the assumption that quaternions are always fully keyed in a pose
asset. Even though quaternions really should be fully keyed, better not
make Blender crash when this assumption doesn't hold.
If the quaternion is only partially keyed, the other components are taken
to be from the unit quaternion (`[1, 0, 0, 0]`) and the result is
normalized before blending into the existing pose.
This fixesblender/blender-addons#104591, which was reported against the
pose library add-on, but actually was caused by an issue in Blender.
f3be425e97