Fix Firefox v47 error if poster="" see BT#11431

Error was "Error loading this resource".
See: https://github.com/johndyer/mediaelement/issues/1769

main/inc/lib/javascript/ckeditor/plugins/video/dialogs/video.js

@@ -9,6 +9,10 @@ CKEDITOR.dialog.add( 'video', function ( editor )
 		if ( !value && this.id=='id' )
 			value = generateId();
 
+        if (value == '') {
+            return;
+        }
+
 		videoNode.setAttribute( this.id, value);
 
 		if ( !value )