Massless Kerr black hole

Kerr metric has the following form:

$$
ds^2 = -left(1 – frac{2GMr}{r^2+a^2cos^2(theta)}right) dt^2 +
left(frac{r^2+a^2cos^2(theta)}{r^2-2GMr+a^2}right) dr^2 +
left(r^2+a^2cos(theta)right) dtheta^2
+ left(r^2+a^2+frac{2GMra^2}{r^2+a^2cos^2(theta)}right)sin^2(theta) dphi^2 –
left(frac{4GMrasin^2(theta)}{r^2+a^2cos^2(theta)}right) dphi, dt
$$

This metric describes a rotating black hole.

If one considers $M=0$:

$$
ds^2 = – dt^2 +
left(frac{r^2+a^2cos^2(theta)}{r^2+a^2}right) dr^2 +
left(r^2+a^2cos(theta)right) dtheta^2
+
left(r^2+a^2right)sin^2(theta) dphi^2
$$

This metric is a solution of the Einstein equations in vacuum.

What is the physical interpretation of such a solution?