[FIX] website_slides: (temporarily) fix infinite loop

Do not fall into an infinite loop when fetching content of an empty webpage
slide in the newly-introduced fullscreen mode.

This commit is linked to task ID 1942836 and PR #31239.

Co-Authored-By: Aurélien Warnon <awa@odoo.com>
Co-Authored-By: Thibault Delavallée <tde@odoo.com>

addons/website_slides/static/src/js/slides_course_fullscreen_player.js

@@ -232,8 +232,10 @@ odoo.define('website_slides.fullscreen', function (require) {
                     'slide_id': self.slide.id
                 }
             }).then(function(data){
-                self.slide.htmlContent = data.html_content;
-                self._renderPlayer();
+                if (data.html_content) {
+                    self.slide.htmlContent = data.html_content;
+                    self._renderPlayer();
+                }
             })
         },
         /**