fix: primary key conflict in upsert

[bbaktaeho]

You do not need to enter the column 'ON CONFLICT' to process 'DO NOTHING' when the Primary Key CONFLICT occurs.

For example,
if you create a partition table using pg_partman extention in PostgreSQL, the parent table might not have a PK.

create table test_table (id int, tt timestamp) partition by range (tt);
create table test_table_template (LIKE test_table);
alter table test_table_template add constraint test_table_pk primary key (id);

select partman.create_parent('temp.test_table', 'tt', 'native', '1 month', p_template_table := 'temp.test_table_template', p_premake := 2);

select constraint_name, constraint_type from information_schema.table_constraints where table_name = 'test_table';
-- no exists primary key
-- +---------------+---------------+
-- |constraint_name|constraint_type|
-- +---------------+---------------+

Create a parent table as above and inject pk into the partitioning table through the template table.
As a result, there is no PK in the parent table.

-- success
insert into test_table (id, tt) values (1, now());

-- there is no unique or exclusion constraint matching the ON CONFLICT specification
insert into test_table (id, tt)
values (1, now()) on conflict (id) do nothing ;

-- success
insert into test_table (id, tt)
values (1, now()) on conflict do nothing ;

So, it looks better to use diesel on_conflict_do_nothing() when it's PK CONFLICT.