Fix invalid logout URL bug caused by empty $shib_AssertionConsumerServiceURL option

If the $shib_AssertionConsumerServiceURL wasn't set or it was empty, the variable
didn't initialized properly when we assembled the logout url.

Fix bug: in a previous commit (82e34a0a), $shib_WAYF unintendedly replaced
when we assembled the login URL so the configuration variable had no effect
(credits to @scolytus, thanks!)