public List<String> getRecipients() {
        return hasAttributeValue() ? getAttributeValue() : ObjectUtils.defaultIfNull(getTo(), List.of());
    }
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/sms/SmsRequest.java