How to provide the key value in a join when a match is not found
Use COALESCE

Purpose
COALESCE returns the first non-null expr in the expression list. You must specify at least two expressions. If all occurrences of expr evaluate to null, then the function returns null.

select c.customer, c.version, coalesce(u.name, to_char(c.userid)) as user c.description
from customerlog c
     left join users u on u.userid = c.userid
where c.customer='Orange'