MAGETWO-94807: Shop By button is not working in a mobile theme

- Updated logic to not call parentElement on empty object
- Updated activate logic to exit upon options disabled
- Updated docBlock to have return value of void

Author: Hwashiang Yu

lib/web/mage/collapsible.js

@@ -442,17 +442,23 @@ define([
 
         /**
          * Activate.
+         *
+         * @return void;
          */
         activate: function () {
-            if (!this.options.disabled) {
-                if (this.options.animate) {
-                    this._animate(showProps);
-                } else {
+            if (this.options.disabled) {
+                return;
+            }
+
+            if (this.options.animate) {
+                this._animate(showProps);
+            } else {
+                if (this.content.length) {
                     this._scrollToTopIfVisible(this.content.get(0).parentElement);
-                    this.content.show();
                 }
-                this._open();
+                this.content.show();
             }
+            this._open();
         },
 
         /**