[FIX] mass_mailing: required field

`popup_content` cannot be required since it is only visible if the
option `group_website_popup_on_exit` is activated.

Fixes #11268
opw-671532

addons/mass_mailing/models/mass_mailing.py

@@ -47,7 +47,7 @@ class MassMailingList(osv.Model):
             _get_contact_nbr, type='integer',
             string='Number of Contacts',
         ),
-        'popup_content': fields.html("Website Popup Content", translate=True, required=True, sanitize=False),
+        'popup_content': fields.html("Website Popup Content", translate=True, sanitize=False),
         'popup_redirect_url': fields.char("Website Popup Redirect URL"),
     }