fix: GREATEST/LEAST should ignore null arguments

[spokodev]

Problem

GREATEST and LEAST return null as soon as any single argument is null:

select greatest(1, null, 3);  -- pg-mem: null
select least(2, null, 1);     -- pg-mem: null

Postgres ignores null arguments in these functions, and returns null only when every argument is null:

select greatest(1, null, 3);       -- postgres: 3
select least(2, null, 1);          -- postgres: 1
select greatest(null::int, null);  -- postgres: null

Ref: Postgres conditional expressions (GREATEST/LEAST) - null values in the argument list are ignored, and the result is null only if all arguments are null.

Cause

Both functions were declared without allowNullArguments, so a null argument short-circuited the whole call to null before the implementation ran.

Fix

Set allowNullArguments: true and filter nulls inside the implementation, returning null only when all arguments are null. Unit tests added in function-calls.spec.ts.