BUG #17066: Cache lookup failed when null (unknown) is passed as anycompatiblemultirange

The following bug has been logged on the website:

Bug reference: 17066
Logged by: Alexander Lakhin
Email address: exclusion(at)gmail(dot)com
PostgreSQL version: 14beta1
Operating system: Ubuntu 20.04
Description:

When calling the simple test function:
create function multirange_test(a anycompatiblemultirange)
returns anycompatible as 'select lower($1);' language sql;

with the NULL argument:
SELECT multirange_test(null);

I get a somewhat unusual error:
ERROR: cache lookup failed for type 0

The variation with the anycompatiblerange (without multi):
create function range_test(a anycompatiblerange)
returns anycompatible as 'select lower($1);' language sql;
SELECT range_test(null);

produces:
ERROR: could not determine polymorphic type anycompatiblerange because
input has type unknown