Validation rule, alert when a field is NOT empty

Question

I know how to check if a field is empty and generate an alert if so:

$('#checkout-form').validate({
    rules: {
        first_name: {
            minlength: 2,
            required: true
        },
        ...
    }
});

But what is the opposite of required? I want to check and generate an alert when my field isn't empty.

Answer 1

$('#checkout-form').validate({
  rules: {
    first_name: {
      minlength: 2,
      required: true,
      equalToParam: ""
    },

But 'Why in the name of all that is holy would you want to do that?'