Native-transformations should support an alternative operator to support null columns

[Seddryck]

This feature is useful when you've some transformations but you are not sure that some of the columns used in the transformation always exist or you fear that the value could be (null)

The syntax [foo] ?? [bar] ?? default | text-to-upper will uppercase the value of the column named foo except if this column doesn't exist or is null where the column bar will be used, again if this column is missing or null then the value default will be uppercased.