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

src/test/regress/expected/subscription.out

@@ -147,6 +147,13 @@ DROP SUBSCRIPTION regress_testsub;
 ERROR:  DROP SUBSCRIPTION cannot run inside a transaction block
 COMMIT;
 ALTER SUBSCRIPTION regress_testsub SET (slot_name = NONE);
+\dRs+
+                                                      List of subscriptions
+      Name       |           Owner            | Enabled |     Publication     | Synchronous commit |           Conninfo           
+-----------------+----------------------------+---------+---------------------+--------------------+------------------------------
+ regress_testsub | regress_subscription_user2 | f       | {testpub2,testpub3} | local              | dbname=regress_doesnotexist2
+(1 row)
+
 -- now it works
 BEGIN;
 DROP SUBSCRIPTION regress_testsub;

src/test/regress/sql/subscription.sql

@@ -109,6 +109,8 @@ COMMIT;
 
 ALTER SUBSCRIPTION regress_testsub SET (slot_name = NONE);
 
+\dRs+
+
 -- now it works
 BEGIN;
 DROP SUBSCRIPTION regress_testsub;