HFP-1779 Allow false for full-screen extension on init

The Video CoP does allow the full-screen extension to be false.
Cmp. https://github.com/liveaspankaj/xapi-video-cop/blob/master/statement_data_model.md

scripts/x-api.js

@@ -94,7 +94,7 @@ H5P.VideoXAPI = (function ($) {
       volume = formatFloat(volume);
 
       var extensions = {};
-      if (typeof isFullscreen !== 'undefined' && isFullscreen) {
+      if (typeof isFullscreen !== 'undefined') {
         extensions['https://w3id.org/xapi/video/extensions/full-screen'] = isFullscreen;
       }
       if (typeof screenSize !== 'undefined') {