Fix table rewrites that include a column without a default.
In
c2fe139c201c I made ATRewriteTable() use tuple slots. Unfortunately
I did not notice that columns can be added in a rewrite that do not
have a default, when another column is added/altered requiring one.
Initialize columns to NULL again, and add tests.
Bug: #16038
Reported-By: anonymous
Author: Andres Freund
Discussion: https://postgr.es/m/16038-
5c974541f2bf6749@postgresql.org
Backpatch: 12, where the bug was introduced in
c2fe139c201c