[FIX] web_editor: fix CTA button upgrade

After upgrading (specifically following this [upgrade PR]), when editing
an element, the editor may be tricked into considering that the page
isn't correct even though it is. It causes the "discard" dialog to open.

Steps to reproduce after the [upgrade PR]:
- Remove the CTA in 16.0 with the bin icon (on the button itself, or
from the edit panel, next to the "Block" section).
- Upgrade to 17.0.
- The button is back. Edit the header and click to hide it.
=> a dialog opens "Are you sure you want to discard your changes?".
- Click either "yes" or "cancel".
=> either way, nothing is discarded nor cancelled.

[upgrade PR]: https://github.com/odoo/upgrade/pull/5500

Related to opw-3644220

closes odoo/odoo#151291

Signed-off-by: Guillaume Dieleman (gdi) <gdi@odoo.com>

addons/web_editor/static/src/js/wysiwyg/wysiwyg.js

@@ -2643,7 +2643,7 @@ export class Wysiwyg extends Component {
                         .removeClass(id)
                         .popover({
                             trigger: 'hover',
-                            content: response.message.data.message || '',
+                            content: response.message.data?.message || '',
                             placement: 'auto',
                         })
                         .popover('show');