Fix value check in page-to-extension-messaging example (#229)

Check the value of "event.data" before dereferencing it,
to avoid errors when void messages are sent using postMessage.

And remove the "event.data.direction" truthness check, because the
actual value is already tested at the next line.

page-to-extension-messaging/content-script.js

@@ -4,7 +4,7 @@ If the message was from the page script, show an alert.
 */
 window.addEventListener("message", function(event) {
   if (event.source == window &&
-      event.data.direction &&
+      event.data &&
       event.data.direction == "from-page-script") {
     alert("Content script received message: \"" + event.data.message + "\"");
   }