handle zero light count

src/mol-gl/shader/chunks/light-frag-params.glsl.ts

@@ -8,8 +8,10 @@
  */
 
 export const light_frag_params = `
+#if dLightCount != 0
     uniform vec3 uLightDirection[dLightCount];
     uniform vec3 uLightColor[dLightCount];
+#endif
 uniform vec3 uAmbientColor;
 
 struct PhysicalMaterial {