Bogus NULL object_name from pg_event_trigger_dropped_objects()

I chanced to experiment with the event trigger function shown in
9.29.2. Processing Objects Dropped by a DDL Command [1].
I got

et=# create table t1(f1 text primary key);
CREATE TABLE
et=# drop table t1;
NOTICE:  DROP TABLE dropped object: table public.t1 public.t1
NOTICE:  DROP TABLE dropped object: type public.t1 public.t1
NOTICE:  DROP TABLE dropped object: type public._t1 public.t1[]
NOTICE:  DROP TABLE dropped object: toast table pg_toast.pg_toast_38092 pg_toast.pg_toast_38092
NOTICE:  DROP TABLE dropped object: index pg_toast.pg_toast_38092_index pg_toast.pg_toast_38092_index
NOTICE:  DROP TABLE dropped object: table constraint public.<NULL> t1_pkey on public.t1
NOTICE:  DROP TABLE dropped object: index public.t1_pkey public.t1_pkey
DROP TABLE
et=#

Note the faulty output for the pkey constraint.  I've not looked
for the cause.

            regards, tom lane

[1] https://www.postgresql.org/docs/current/functions-event-triggers.html#PG-EVENT-TRIGGER-SQL-DROP-FUNCTIONS