Correctly mark pg_subscription.subslotname as nullable.

Due to the layout of this catalog, subslotname has to be explicitly
marked BKI_FORCE_NULL, else initdb will default to the assumption
that it's non-nullable.  Since, in fact, CREATE/ALTER SUBSCRIPTION
will store null values there, the existing marking is just wrong,
and has been since this catalog was invented.

We haven't noticed because not much in the system actually depends
on attnotnull being truthful.  However, JIT'ed tuple deconstruction
does depend on that in some cases, allowing crashes or wrong answers
in queries that inspect pg_subscription.  Commit 9de77b54 quite
accidentally exposed this on the buildfarm members that force JIT
activation.

Back-patch to v13.  The problem goes further back, but we cannot
force initdb in released branches, so some klugier solution will
be needed there.  Before working on that, push this simple fix
to try to get the buildfarm back to green.

Discussion: https://postgr.es/m/4118109.1595096139@sss.pgh.pa.us

doc/src/sgml/catalogs.sgml

@@ -7529,7 +7529,8 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
       </para>
       <para>
        Name of the replication slot in the upstream database (also used
-       for the local replication origin name)
+       for the local replication origin name);
+       null represents <literal>NONE</literal>
       </para></entry>
      </row>
 

src/include/catalog/catversion.h

@@ -53,6 +53,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	202007181
+#define CATALOG_VERSION_NO	202007192
 
 #endif

src/include/catalog/pg_subscription.h

@@ -56,7 +56,7 @@ CATALOG(pg_subscription,6100,SubscriptionRelationId) BKI_SHARED_RELATION BKI_ROW
 	text		subconninfo BKI_FORCE_NOT_NULL;
 
 	/* Slot name on publisher */
-	NameData	subslotname;
+	NameData	subslotname BKI_FORCE_NULL;
 
 	/* Synchronous commit setting for worker */
 	text		subsynccommit BKI_FORCE_NOT_NULL;