When theta=0, you have
q = cos(0) + sin(0)*(ai+bj+ck) = 1 + 0 = 1
That means applying the "rotation" to the vector gives
v' = q * v * q^-1 = 1 * v * 1^-1 = v
So the output ("rotated") vector is the same as the input, as you would expect for a rotation by 0.
