Update information_schema definition for not-null constraints

Update information_schema definition for not-null constraints

Now that we have catalogued not-null constraints, our information_schema
definition can be updated to grab those rather than fabricate synthetic
definitions.

Note that we still don't have catalog rows for not-null constraints on
domains, but we've never had not-null constraints listed in
information_schema, so that's a problem to be solved separately.

Co-authored-by: Peter Eisentraut <peter.eisentraut@enterprisedb.com>
Co-authored-by: Álvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://postgr.es/m/81b461c4-edab-5d8c-2f88-203108425340@enterprisedb.com
Discussion: https://postgr.es/m/202309041710.psytrxlsiqex@alvherre.pgsql