Add tab-completion for ALTER TABLE not-nulls

Add tab-completion for ALTER TABLE not-nulls

The command is: ALTER TABLE x ADD [CONSTRAINT y] NOT NULL z

This syntax was added in 18, but I got pushback for getting commit
dbf42b84ac7b in 18 (also tab-completion for new syntax) after the
feature freeze, so I'll put this in master only for now.

Author: Álvaro Herrera <alvherre@kurilemu.de>
Reported-by: Fujii Masao <masao.fujii@oss.nttdata.com>
Reviewed-by: Fujii Masao <masao.fujii@oss.nttdata.com>
Reviewed-by: Dagfinn Ilmari Mannsåker <ilmari@ilmari.org>
Discussion: http://postgr.es/m/d4f14c6b-086b-463c-b15f-01c7c9728eab@oss.nttdata.com
Discussion: http://postgr.es/m/202505111448.bwbfomrymq4b@alvherre.pgsql