Disallow NO INHERIT not-null constraints on partitioned tables

Disallow NO INHERIT not-null constraints on partitioned tables

Such constraints are semantically useless and only bring weird cases
along, so reject them.

As a side effect, we can no longer have "throwaway" constraints in
pg_dump for primary keys in partitioned tables, but since they don't
serve any useful purpose, we can just omit them.

Maybe this should be done for all types of constraints, but it's just
not-null ones that acquired this "ability" in the 17 timeframe, so for
the moment I'm not changing anything else.

Per note by Alexander Lakhin.
Discussion: https://postgr.es/m/7d923a66-55f0-3395-cd40-81c142b5448b@gmail.com