_LightMatrix0 and Directional Light

I am trying to access the _LightMatrix0 in a forward base pass to calculate some light point of view coordinate but it seems that the _LightMatrix0 is empty with directional light.

Any idea how can I solve this and how could I access to the _LightMatrix0 with directional light (without cookies) ?

I am trying to do this kind of thing.

 float4 position = mul(_LightMatrix0, worldPos);