PicketBoxBasedIdentity.exists() should check if a valid JAAS Subject exists instead of always returning true

The RealmIdentity.exists() method should be used to verify if a valid identity exists before an attempt to call other non-authentication methods - e.g. getAuthorizationIdentity() - is made.

The PicketBoxBasedIdentity implementation in the SecurityDomainContextRealm is erroneously returning true when in fact it should be checking if a valid Subject was established as part of a previous JAAS authentication.

The getAuthorizationIdentity() method can then simply throw an Exception if it is called without a valid JAAS Subject in place. Client code should check the result of the exists() method before attempting to get an AuthorizationIdentity so any code invoking getAuthorizationIdentity() without checking first if a valid identity exists should fail.