Advanced session hijacking mitigation

utilizing ssl_session_id (when mod_ssl is enabled) and ip/user-agent
when using raw http connections an additional check is done to ensure
that cooked are not copied between systems thus preventing session
hijacking

If a user with a valid CAS session attempts to copy another user's
cookie the existing CAS session will simply reestablish and the stolen
cookie will be invalidated.
While this could potentially create a denial of service; the risk is
limited to non-https connections.

In addition the isSSL function now utilizes the mod_ssl optional function ssl_is_https 
Is there a reason to use [NULL] instead of treating the empty string as a null session ID and "(void *) 0" in its place?