code: function ifDefinedDirective(value) {
    return function(part) {
      if (value === undefined && isAttributePart(part)) {
        return part.setValue(nothingString);
      }
      part.setValue(value);
    };
  }
docs: Sets the attribute if 'value' is defined,
  removes the attribute if undefined.
  
  @param { any } value
  @returns { (part: AttributePart) => void }