Correctly enable/disable the sky light

Assets/ScriptableRenderLoop/HDRenderLoop/HDRenderLoop.cs

             //Shader.SetGlobalTexture("_CookieTextures", m_CookieTexArray.GetTexCache());
             //Shader.SetGlobalTexture("_CubeCookieTextures", m_CubeCookieTexArray.GetTexCache());
             Shader.SetGlobalTexture("_EnvTextures", m_CubeReflTexArray.GetTexCache());
-            m_SkyRenderer.SetGlobalSkyTexture();
+
+            if (m_SkyRenderer.IsSkyValid(m_SkyParameters))
+            {
+                m_SkyRenderer.SetGlobalSkyTexture();
+                Shader.SetGlobalInt("_EnvLightSkyEnabled", 1);
+            }
+            else
+            {
+                Shader.SetGlobalInt("_EnvLightSkyEnabled", 0);
+            }
 
             if (debugParameters.useSinglePassLightLoop)
             {

Assets/ScriptableRenderLoop/HDRenderLoop/Lighting/SinglePass/SinglePass.hlsl

     uint _PunctualLightCount;
     uint _AreaLightCount;
     uint _EnvLightCount;
+    int  _EnvLightSkyEnabled;         // TODO: make it a bool and move it into '_EnvLightSky'
-    float4 _DirShadowSplitSpheres[4]; // TODO share this max between C# and hlsl
+    float4 _DirShadowSplitSpheres[4]; // TODO: share this max between C# and hlsl
 CBUFFER_END
 
 struct LightLoopContext

Assets/ScriptableRenderLoop/HDRenderLoop/Lighting/SinglePass/SinglePassLoop.hlsl

         iblSpecularLighting = lerp(iblSpecularLighting, localSpecularLighting, weight.y);
     }
 
-    // Sky Ibl
+    // Only apply sky IBL if the sky texture is available.
+    if (_EnvLightSkyEnabled)
     {
         float3 localDiffuseLighting, localSpecularLighting;
         float2 weight;

Assets/ScriptableRenderLoop/HDRenderLoop/Sky/SkyRenderer.cs

 
         }
 
-        bool IsSkyValid(SkyParameters parameters)
+        public bool IsSkyValid(SkyParameters parameters)
         {
             // Later we will also test shader for procedural skies.
             return parameters.skyHDRI != null;