Make setting stroke color and width optional

src/objectliterals.jsdoc

@@ -638,8 +638,8 @@
 
 /**
  * @typedef {Object} ol.style.StrokeOptions
- * @property {ol.Color|string} color Color.
- * @property {number} width Width.
+ * @property {ol.Color|string|undefined} color Color.
+ * @property {number|undefined} width Width.
  * @todo stability experimental
  */
 

src/ol/style/strokestyle.js

@@ -13,10 +13,10 @@ ol.style.Stroke = function(options) {
   /**
    * @type {ol.Color|string}
    */
-  this.color = options.color;
+  this.color = goog.isDef(options.color) ? options.color : null;
 
   /**
-   * @type {number}
+   * @type {number|undefined}
    */
   this.width = options.width;
 };