Merge pull request #671 from Unity-Technologies/volume-fixes3

Missing null check

ScriptableRenderPipeline/Core/Volume/VolumeParameter.cs

 
         public static bool operator ==(VolumeParameter<T> lhs, T rhs)
         {
-            return lhs.value.Equals(rhs);
+            return lhs.value != null && lhs.value.Equals(rhs);
         }
 
         public static bool operator !=(VolumeParameter<T> lhs, T rhs)