Add a compile-time switch between the full and the empty VolumetricLighting.compute shader

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Lighting/Volumetrics/Resources/VolumetricLighting.compute

 #define GROUP_SIZE_1D 16
 #define GROUP_SIZE_2D (GROUP_SIZE_1D * GROUP_SIZE_1D)
 
+#if (SHADEROPTIONS_VOLUMETRIC_LIGHTING_PRESET != 0) // Switch between the full and the empty shader
+
 //--------------------------------------------------------------------------------------------------
 // Included headers
 //--------------------------------------------------------------------------------------------------
 
     FillVolumetricLightingBuffer(context, featureFlags, posInput, ray);
 }
+
+#else
+
+[numthreads(GROUP_SIZE_2D, 1, 1)]
+void VolumetricLighting(uint2 groupId       : SV_GroupID,
+                        uint  groupThreadId : SV_GroupThreadID)
+{
+    // Reduce compile times if the feature is disabled.
+}
+
+#endif // SHADEROPTIONS_VOLUMETRIC_LIGHTING_PRESET