Assert that we don't insert nulls into attnotnull catalog columns.

 Assert that we don't insert nulls into attnotnull catalog columns.
 
 The executor checks for this error, and so does the bootstrap catalog
 loader, but we never checked for it in retail catalog manipulations.
 The folly of that has now been exposed, so let's add assertions
 checking it.  Checking in CatalogTupleInsert[WithInfo] and
 CatalogTupleUpdate[WithInfo] should be enough to cover this.
 
 Back-patch to v10; the aforesaid functions didn't exist before that,
 and it didn't seem worth adapting the patch to the oldest branches.
 But given the risk of JIT crashes, I think we certainly need this
 as far back as v11.
 
 Pre-v13, we have to explicitly exclude pg_subscription.subslotname
 and pg_subscription_rel.srsublsn from the checks, since they are
 mismarked.  (Even if we change our mind about applying BKI_FORCE_NULL
 in the branch tips, it doesn't seem wise to have assertions that
 would fire in existing databases.)
 
 Discussion: https://postgr.es/m/298837.1595196283@sss.pgh.pa.us