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