Fixed a missing Render Target clear in the sky system that caused NaNs on some platform when no sky is specified.

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Sky/SkyRenderingContext.cs

             {
                 if (skyContext.skyParametersHash != 0)
                 {
+                    CoreUtils.ClearCubemap(cmd, m_SkyboxCubemapRT, Color.black, true);
                     if (m_SupportsConvolution)
                     {
                         CoreUtils.ClearCubemap(cmd, m_SkyboxGGXCubemapRT, Color.black, true);