private String principalClaimName() {
        String principalClaimNameValue = option(PRINCIPAL_CLAIM_NAME_OPTION);
        return Utils.isBlank(principalClaimNameValue) ? "sub" : principalClaimNameValue.trim();
    }
Return true if this instance has been configured, otherwise false

@return true if this instance has been configured, otherwise false
clients/src/main/java/org/apache/kafka/common/security/oauthbearer/internals/unsecured/OAuthBearerUnsecuredValidatorCallbackHandler.java