oracle script defaulting a variable

How is s SQL script variable tested for not being defined and if not defined  set a default value.

A very simple example to describe the issue.  In the third input example I want the missing &3 to default to *. The real problem is much more complicated, this simply defineds the issue.

@test_script T_user kee *
@test_script T_user kee user_id
@test_script T_user kee

--------------------------------------------
-- test_script.sql
define col_name=&3
select &col_name from &1  where user_id like '%&2%';

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what I would like to have happen

if &3 is null then
     define col_name=*
else
   define col_name=&3
end if;

select &col_name from &1  where user_id like '%&2%';