Re: pg_subscription.subslotname is wrongly marked NOT NULL

I wrote:
>> It's also a bit annoying that we have no mechanized checks that
>> would catch this inconsistency. If JIT is going to be absolutely
>> dependent on NOT NULL markings being accurate, we can't really
>> have such a laissez-faire attitude to C code getting it wrong.

> It seems like at least in assert-enabled builds, we'd better have
> a cross-check for that. I'm not sure where's the best place.

I concluded that we should put this into CatalogTupleInsert and
CatalogTupleUpdate. The bootstrap data path already has a check
(see InsertOneNull()), and so does the executor, so we only need
to worry about tuples that're built manually by catalog manipulation
code. I think all of that goes through these functions. Hence,
as attached.

... and apparently, I should have done this task first, because
damn if it didn't immediately expose another bug of the same ilk.
pg_subscription_rel.srsublsn also needs to be marked nullable.

regards, tom lane