The presence of a NULL "defaclacl" value in pg_default_acl prevents the dropping of a role.

Hello postgres hackers:
I recently came across a scenario involving system catalog "pg_default_acl"
where a tuple contains a NULL value for the "defaclacl" attribute. This can cause
confusion while dropping a role whose default ACL has been changed.
Here is a way to reproduce that:

``` example
postgres=# create user adminuser;
CREATE ROLE
postgres=# create user normaluser;
CREATE ROLE
postgres=# alter default privileges for role adminuser grant all on tables to normaluser;
ALTER DEFAULT PRIVILEGES
postgres=# alter default privileges for role adminuser revoke all ON tables from adminuser;
ALTER DEFAULT PRIVILEGES
postgres=# alter default privileges for role adminuser revoke all ON tables from normaluser;
ALTER DEFAULT PRIVILEGES
postgres=# select * from pg_default_acl where pg_get_userbyid(defaclrole) = 'adminuser';
oid | defaclrole | defaclnamespace | defaclobjtype | defaclacl
-------+------------+-----------------+---------------+-----------
16396 | 16394 | 0 | r | {}
(1 row)
postgres=# drop user adminuser ;
ERROR: role "adminuser" cannot be dropped because some objects depend on it
DETAIL: owner of default privileges on new relations belonging to role adminuser
```

I believe this is a bug since the tuple can be deleted if we revoke from "normaluser"
first. Besides, according to the document:
https://www.postgresql.org/docs/current/sql-alterdefaultprivileges.html

If you wish to drop a role for which the default privileges have been altered,
it is necessary to reverse the changes in its default privileges or use DROP OWNED BY
to get rid of the default privileges entry for the role.

There must be a way to "reverse the changes", but NULL value of "defaclacl"
prevents it. Luckily, "DROP OWNED BY" works well.

The code-level reason could be that the function "SetDefaultACL" doesn't handle
the situation where "new_acl" is NULL. So I present a simple patch here.

diff --git a/src/backend/catalog/aclchk.c b/src/backend/catalog/aclchk.c
index 01ff575b093..0e313526b28 100644
--- a/src/backend/catalog/aclchk.c
+++ b/src/backend/catalog/aclchk.c
@@ -1335,7 +1335,7 @@ SetDefaultACL(InternalDefaultACL *iacls)
   */
  aclitemsort(new_acl);
  aclitemsort(def_acl);
- if (aclequal(new_acl, def_acl))
+ if (aclequal(new_acl, def_acl) || ACL_NUM(new_acl) == 0)
  {
   /* delete old entry, if indeed there is one */
   if (!isNew)

Best regards,
Boyu Yang