execute immediate ' ... to_char(:x) ...' using null 

select :x from dual
union all
select :y from dual

SELECT * FROM (
  select :x as Z from dual
    union all
  select :y as Z from dual
) WHERE Z IS NULL

create or replace package tst is
  function f(x number) return number;
  function f(x date) return date;
end;
/

create or replace package body tst is
  function f(x number) return number is
  begin
    return 3.14-nvl(x,1.07);
  end f;
  
  function f(x date) return date is
  begin
    return nvl(x, sysdate) + interval '1' day;
  end f;
end;
/

declare 
  v anydata;
begin
  execute immediate 'select tst.f(:x) xx from dual' into v using null;
--  execute immediate 'select tst.f(:x) xx from dual' into v using '20181005';
end;
/