The dot product is a·b = aₓbₓ+aᵧbᵧ+a_zb_z = |a||b|cos θ. Two nonzero vectors are perpendicular when their dot product is zero. A zero vector has no direction, so it cannot define an angle.
The dot product is a·b = aₓbₓ+aᵧbᵧ+a_zb_z = |a||b|cos θ. Two nonzero vectors are perpendicular when their dot product is zero. A zero vector has no direction, so it cannot define an angle.