BUG #16945: where value in (null) set results inconsistent

The following bug has been logged on the website:

Bug reference:      16945
Logged by:          Glenn Widener
Email address:      glennwidener@gmail.com
PostgreSQL version: 11.5
Operating system:   Windows 10
Description:

Pursuant to a fix in Hibernate, that uses "where value in (null)" as a
workaround for several DB's not handling "where value in ()":
https://hibernate.atlassian.net/browse/HHH-8091

Consider this inconsistency:
test-db=> select id from project_type_variants where id in (0);
 id
----
(0 rows)

test-db=> select id from project_type_variants where id in (null);
 id
----
(0 rows)

test-db=> select id from project_type_variants where id not in (0);
 id
----
  1
  2
  3
(3 rows)

test-db=> select id from project_type_variants where id not in (null);
 id
----
(0 rows)

Needs to work consistently, because (null) is a required workaround for:

test-db=> select id from project_type_variants where id in ();
ERROR:  syntax error at or near ")"