Kluge slot_compile_deform() to ignore incorrect attnotnull markings.
Since we mustn't force an initdb in released branches, there is no
simple way to correct the markings of pg_subscription.subslotname
and pg_subscription_rel.srsublsn as attnotnull in existing pre-v13
installations.
Fortunately, released branches don't rely on attnotnull being correct
for much. The planner looks at it in relation_excluded_by_constraints,
but it'd be difficult to get that to matter for a query on a system
catalog. The only place where it's really problematic is in JIT's
slot_compile_deform(), which can produce incorrect code that crashes
if there are NULLs in an allegedly not-null column.
Hence, hack up slot_compile_deform() to be specifically aware of
these two incorrect markings and not trust them.
This applies to v11 and v12; the JIT code didn't exist before that,
and we've fixed the markings in v13.
Discussion: https://postgr.es/m/229396.
1595191345@sss.pgh.pa.us