[FIX] mass_mailing: mail content not transferring to mail body

Prior this commit:
While sending mail through CRM, the mail body doesn't show message content
in the mass_mailing module.

After this commit:
User will be able to see the message content inside Mail Body.

Task-3245572

closes odoo/odoo#123963

X-original-commit: a4549981790a6aa85ff59ea5ec6157c3ec3cfda6
Signed-off-by: Antoine Guenet (age) <age@odoo.com>
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>

addons/mass_mailing/static/src/js/mass_mailing_html_field.js

@@ -601,6 +601,15 @@ export class MassMailingHtmlField extends HtmlField {
     async _getWysiwygClass() {
         return getWysiwygClass({moduleName: 'mass_mailing.wysiwyg'});
     }
+    /**
+     * @override
+     */
+    async _setupReadonlyIframe() {
+        if (!this.props.value.length) {
+            this.props.value = this.props.record.data.body_html;
+        }
+        await super._setupReadonlyIframe();
+    }
 }
 
 MassMailingHtmlField.props = {