[FIX] web_editor: wrong display for td in Apple Mail

the td are displayed with wrong background color if the td tag is empty

addons/web_editor/static/src/js/editor/transcoder.js

@@ -229,6 +229,9 @@ function classToStyle($editable) {
         } else {
             $target.attr('style', style);
         }
+        if (this.nodeName === 'TD' && !this.childNodes.length) {
+            this.innerHTML = ' ';
+        }
     });
 }