Transaction local custom settings set to '' rather than removed entirely after transaction ends

Before creating a custom local setting, current_setting(string) will raise
an error:

select current_setting('foo.bar');
ERROR: unrecognized configuration parameter "foo.bar"

After assigning a local value during a transaction and finishing the
transaction, the setting becomes recognised, has an empty string value:

begin;

set local "foo.bar" to 'baz';

rollback;

select current_setting('foo.bar');

current_setting

select pg_typeof(current_setting('foo.bar'));

pg_typeof

text

This behaviour lasts for the duration of the connection, and returns to the
initial state after reconnecting.

This caused me problems, as I was using the new 'missing_ok' parameter for
current_setting (added in PostgreSQL 9.6) to return null, e.g.

select current_setting('foo.bar.baz', true);
NULL

I am using local settings to store an (e.g. application user UUID), which I
then use in RLS policies and also refer to in a DEFAULT clause for a table
column, e.g.

create table doc (
id uuid primary key,
title text,
author uuid default current_setting('app.user.id', true)::uuid
);

However app.user.id is only set if a user is performing a query, it's also
possible for a app service role to connect to the database and insert rows
too. For this to work I rely on current_setting returning null (rather than
an empty string).

I would like the presence of a local setting to not leak out of a
transaction.

--
Cheers,
Brad