Correct typedef of selectinteraction options obj

The key `features` of the constructor options that can be passed to
`ol.interaction.Select` can also be `undefined`.

externs/olx.js

@@ -2995,7 +2995,7 @@ olx.interaction.SelectOptions.prototype.multi;
  * not set the interaction will create a collection. In any case the collection
  * used by the interaction is returned by
  * {@link ol.interaction.Select#getFeatures}.
- * @type {ol.Collection.<ol.Feature>}
+ * @type {ol.Collection.<ol.Feature>|undefined}
  * @api
  */
 olx.interaction.SelectOptions.prototype.features;