onValidation.js

import("system.translate");
import("system.result");
import("Sql_lib");
import("system.vars");

var contactId = vars.get("$field.CONTACT_ID");

var standardAddress = newSelect("CONTACT.ADDRESS_ID")
                                .from("CONTACT")
                                .where("CONTACT.CONTACTID", contactId)
                                .cell();

if(!standardAddress)
{
    result.string(translate.text("The selected Contact doesn't have an address set."));
}