BUG #17066: Cache lookup failed when null (iso-8859-1) is passed as anycompatiblemultirange

The following bug has been logged on the website:

Bug reference:      17066
Logged by:          Alexander Lakhin
Email address:      exclusion@gmail.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