public String getRecipient() {
        return FunctionUtils.doIf(hasAttributeValue(),
            () -> getAttributeValue().map(Object::toString).orElseGet(this::getTo),
            this::getTo).get();
    }
Gets recipients by first using the attribute
from the principal. If none found or empty value,
uses the provided address.

@return the recipients
core/cas-server-core-notifications-api/src/main/java/org/apereo/cas/notifications/call/PhoneCallRequest.java