default String resolve() {
        return UNKNOWN_USER;
    }
Called when there is no other way to resolve the principal (i.e. an error was captured, auditing was not
called, etc.)
<p>
Note, this method should NEVER throw an exception *unless* the expectation is that a failed resolution causes
the entire transaction to fail.  Otherwise use {@link PrincipalResolver#UNKNOWN_USER}.

@return the principal.  CANNOT be NULL.
api/cas-server-core-api-audit/src/main/java/org/apereo/inspektr/common/spi/PrincipalResolver.java