How should the Builder Pattern handle undefined optionals?

I'm constructing a Builder in JavaScript, and I'm not sure how builders normally handle undefined values for optionals. I would like to think that the Builder doesn't append the optional field to the object if the field is undefined. Here is a sample:

Builder:

function Thing(required1, required2, required3) {
 //check required params 

  var fields = {
    required1: required1, 
    required2: required2, 
    required3: required3
  };  

  var _withOptionalParam = function(param) {
    if(!param) { return this; } //exit function early if param is undefined

    fields.param = param; 
    return this; 
  };

  var builder = {
    withOptionalParam: _withOptionalParam 
  };

  return builder;
}

In action:

var thing = new Thing("foo","bar","baz").withOptionalParam(undefined);

//'thing' should be 
{ 
  required1:"foo", 
  required2:"bar", 
  required3:"baz"
};

Thanks in advance!