Re: You might be able to move the set-returning function into aLATERAL FROM item.

I think you could push the conditionality into a plpgsql function,
something like (untested)

create function jsonb_elements_if_array(j jsonb) returns setof jsonb as $$
begin
  if jsonb_typeof(j) = 'array' then
    return query select jsonb_array_elements(j);
  end if;
end$$
strict immutable language plpgsql;

Note that this gives *no* elements, rather than a single NULL value,
if the input isn't an array --- but that seems to me to make more sense
than your existing code anyhow.  If you disagree, add "else return next
null::jsonb".