Fix #144918: World volume with zero density not rendering correctly

OneAPI has some problem with `exp(-0 * FLT_MAX)`.

Pull Request: https://projects.blender.org/blender/blender/pulls/144979

intern/cycles/kernel/integrator/shade_volume.h

@@ -596,7 +596,8 @@ ccl_device Spectrum volume_transmittance(KernelGlobals kg,
       sd->P = ray->P + ray->D * shade_t;
       tau_k += volume_shader_eval_extinction<shadow>(kg, state, sd, path_flag);
     }
-    return exp(-tau_k * step_size);
+    /* OneAPI has some problem with exp(-0 * FLT_MAX). */
+    return is_zero(tau_k) ? one_spectrum() : exp(-tau_k * step_size);
   }
 
   /* Estimations of optical thickness. */