That all makes sense.
I hadn’t really considered the alternatives - what would happen to the nulls, or where they would go otherwise.
In other cases where nulls have been cast to 0, like the arithmetic operators and boolean operators , there was a clear and preferable alternative. As far as alternatives to this problem go, none of the options you suggest (first, last, or leave in place) seem conclusively better. At least the cast to zero behavior is predictable.
Thanks!