[FIX] mass_mailing: issue when pasting html in email marketing

Before this commit:

When you try to copy paste html in code view of email marketing then in preview
it will show the code instead of template.

After this commit:

Now when you try to paste html in code view of email marketing then it will show
template preview as a result.

task-3473633

X-original-commit: 5cd1a900d89c34877be2c1036ad52d8b4a70c849
Part-of: odoo/odoo#139384

addons/mass_mailing/static/src/js/mass_mailing_wysiwyg.js

@@ -101,7 +101,7 @@ export class MassMailingWysiwyg extends Wysiwyg {
         const initialDropZone = this.$editable[0].querySelector('.o_mail_wrapper_td');
         const parsedHtml = new DOMParser().parseFromString(currentValue, "text/html");
         if (initialDropZone && !parsedHtml.querySelector('.o_mail_wrapper_td')) {
-            initialDropZone.replaceChildren(currentValue);
+            initialDropZone.replaceChildren(...parsedHtml.body.childNodes);
         } else {
             super.setValue(...arguments);
         }