sql: `pg_catalog.pg_get_viewdef` broken for materialized views

Needed for #88061.

**Describe the problem**

`pg_get_viewdef()` returns NULL on materialized views.
In PostgreSQL, it returns the view definition.

**To Reproduce**

```sql
create materialized view foo as select 123;
select pg_get_viewdef(oid) from pg_class c where c.relname = 'foo';
```

cc @rafiss for triage

Jira issue: CRDB-19693
Epic: CRDB-23454