Clamp NaNs arising during light sampling

Assets/ScriptableRenderLoop/ShaderLibrary/ImageBasedLighting.hlsl

         }
     }
 
+    // Prevent NaNs arising from the division of 0 by 0.
+    cbsdfInt = max(cbsdfInt, FLT_MIN);
+
     return float4(lightInt / cbsdfInt, 1.0);
 }