private static boolean isEmpty(PrincipalCollection pc) {
        return pc == null || pc.isEmpty();
    }
Throws ShortCircuitIterationException if stopAfterFirstSuccess is set and authentication is
successful with a previously consulted realm.
Returns the <code>aggregate</code> method argument, without modification
otherwise.
core/src/main/java/org/apache/shiro/authc/pam/FirstSuccessfulStrategy.java