BUG #18139: Causing FATAL error on database using function last() or first()

The following bug has been logged on the website:

Bug reference:      18139
Logged by:          Paul Gazagne
Email address:      pyrotarlu74@gmail.com
PostgreSQL version: 14.9
Operating system:   (Ubuntu 11.4.0-1ubuntu1~22.04) 11.4.0, 64-bit
Description:

How to reproduce the error : 

Setup table and dataset : 
CREATE TABLE test (
    id int4 not null,
    value numeric(20, 6) NULL,
    value_t text NULL
);

INSERT INTO test (id)
    VALUES (1);
INSERT INTO test (id,value,value_t)
    VALUES (2,2,'it is 2');
INSERT INTO test (id)
    VALUES (3);
INSERT INTO test (id,value,value_t)
    VALUES (4,4,'it is 4');
INSERT INTO test (id)
    VALUES (5);

If I use the function last/first on this dataset, if the first rows parsed
by the function contain null values, it cause FATAL error on database.

Examples : 
These queries work fine : 

select 
last(value_t,value) as thatsok
from test t
where id between 2 and 4;

select 
last(value_t,value) as thatsok
from test t
where id between 2 and 5;

select 
first(value_t,value) as thatsok
from test t
where id between 2 and 4;

select 
first(value_t,value) as thatsok
from test t
where id between 2 and 5;

These ones cause FATAL error : 

select 
last(value_t,value) as fatal_error
from test t
where id between 1 and 5;

select 
first(value_t,value) as fatal_error
from test t
where id between 1 and 5;