fix: concat_ws should skip null values and honour a null separator

[spokodev]

Problem

concat_ws mishandles nulls three ways:

select concat_ws(',', 'a', null, 'b');  -- pg-mem: 'a,,b'
select concat_ws(null, 'a', 'b');       -- pg-mem: 'anullb'
select concat_ws(',', null, null);      -- pg-mem: ','

Postgres skips null value arguments, returns null when the separator is null, and returns an empty string when every value argument is null:

select concat_ws(',', 'a', null, 'b');  -- postgres: 'a,b'
select concat_ws(null, 'a', 'b');       -- postgres: null
select concat_ws(',', null, null);      -- postgres: ''

Ref: Postgres string functions (concat_ws)

Cause

The implementation was (separator, ...x) => x?.join(separator), which turns each null value into an empty field (the separator is still emitted), coerces a null separator to the string "null", and can never return null.

Fix

Return null when the separator is null, and skip null values before joining (an all-null value list yields an empty string). Unit tests added in function-calls.spec.ts.