`citus_prepare_pg_upgrade` assumes `public` schema exists

Maybe we should create `public` schema ourselves ?, or is this the intended behavior ?:

```sql
create schema my_schema;
set search_path to my_schema;
drop schema public cascade;
create extension citus;
select citus_prepare_pg_upgrade();
NOTICE: aggregate array_cat_agg(anycompatiblearray) does not exist, skipping
NOTICE: schema "public" does not exist, skipping
NOTICE: schema "public" does not exist, skipping
NOTICE: schema "public" does not exist, skipping
NOTICE: schema "public" does not exist, skipping
NOTICE: schema "public" does not exist, skipping
NOTICE: schema "public" does not exist, skipping
NOTICE: schema "public" does not exist, skipping
NOTICE: schema "public" does not exist, skipping
NOTICE: schema "public" does not exist, skipping
NOTICE: schema "public" does not exist, skipping
NOTICE: schema "public" does not exist, skipping
NOTICE: schema "public" does not exist, skipping
ERROR: schema "public" does not exist
CONTEXT: SQL statement "CREATE TABLE public.pg_dist_partition AS SELECT * FROM pg_catalog.pg_dist_partition"
PL/pgSQL function citus_prepare_pg_upgrade() line 32 at SQL statement
```