Fix NRE on Veiw cloning

It will almost never appear in real life but in tests it can be.
Since we extract views anyway cloning should work too

Author: alex-kulakov

Orm/Xtensive.Orm/Orm/Upgrade/Internals/CatalogCloner.cs

@@ -157,7 +157,9 @@ private void CloneViews(Schema newSchema, Schema sourceSchema)
         var newView = newSchema.CreateView(sourceView.Name);
         CopyDbName(newView, sourceView);
         newView.CheckOptions = sourceView.CheckOptions;
-        newView.Definition = (SqlNative) sourceView.Definition.Clone();
+        if (sourceView.Definition != null) {
+          newView.Definition = (SqlNative) sourceView.Definition.Clone();
+        }
         CloneViewColumns(newView, sourceView);
         CloneIndexes(newView, sourceView);
       }