Bug in Configuration Setting Functions after a Commit

Hello:

I'm enthusiastic about the PostgreSQL project and this is my first bug
report. Please apologize if I'm on the wrong channel.

The bug is the following: when I set a custom variable using the
configuration settings functions within a transaction, after I commit the
transaction, instead of the variable becoming null, it becomes an empty
string.

How to reproduce:

SELECT current_setting('custom.user_id', true)::INTEGER; -- NULL

BEGIN;

SELECT set_config('custom.user_id', '2', true);

SELECT current_setting('custom.user_id', true)::INTEGER; -- 2

END;

SELECT current_setting('custom.user_id', true)::INTEGER; -- Empty string
instead of null

PostgreSQL version: PostgreSQL 15.10 (Debian 15.10-0+deb12u1) on
x86_64-pc-linux-gnu, compiled by gcc (Debian 12.2.0-14) 12.2.0, 64-bit